The current `f32|f64.to_degrees` implementation uses a division to calculate 180/π, which causes a loss of precision. Using a constant is still not perfect (implementing a maximally-precise algorithm would come with a high performance cost), but improves precision with a minimal change.

Swapping the conditions generates more efficient x86 assembly. See
https://github.com/rust-lang/rust/pull/46926#issuecomment-354567412.

This commit updates the bootstrap compiler and clears out a number
of #[cfg(stage0)] annotations and related business

Float literals need to be parsed as the correct type so they can be
rounded correctly.

See #42425

See #42423

[unstable, deprecated since 1.11.0]

Although the set of APIs being stabilized this release is relatively small, the
trains keep going! Listed below are the APIs in the standard library which have
either transitioned from unstable to stable or those from unstable to
deprecated.

Stable

* `BTreeMap::{append, split_off}`
* `BTreeSet::{append, split_off}`
* `Cell::get_mut`
* `RefCell::get_mut`
* `BinaryHeap::append`
* `{f32, f64}::{to_degrees, to_radians}` - libcore stabilizations mirroring past
  libstd stabilizations
* `Iterator::sum`
* `Iterator::product`

Deprecated

* `{f32, f64}::next_after`
* `{f32, f64}::integer_decode`
* `{f32, f64}::ldexp`
* `{f32, f64}::frexp`
* `num::One`
* `num::Zero`

Added APIs (all unstable)

* `iter::Sum`
* `iter::Product`
* `iter::Step` - a few methods were added to accomodate deprecation of One/Zero

Removed APIs

* `From<Range<T>> for RangeInclusive<T>` - everything about `RangeInclusive` is
  unstable

Closes #27739
Closes #27752
Closes #32526
Closes #33444
Closes #34152
cc #34529 (new tracking issue)

This adds short summaries to all num consts.

The referenced issues here were both closed, so hook up a new issue which tracks
specifically the prelude traits being unstable.

This is a standard "clean out libstd" commit which removes all 1.5-and-before
deprecated functionality as it's now all been deprecated for at least one entire
cycle.

This commit is the standard API stabilization commit for the 1.6 release cycle.
The list of issues and APIs below have all been through their cycle-long FCP and
the libs team decisions are listed below

Stabilized APIs

* `Read::read_exact`
* `ErrorKind::UnexpectedEof` (renamed from `UnexpectedEOF`)
* libcore -- this was a bit of a nuanced stabilization, the crate itself is now
  marked as `#[stable]` and the methods appearing via traits for primitives like
  `char` and `str` are now also marked as stable. Note that the extension traits
  themeselves are marked as unstable as they're imported via the prelude. The
  `try!` macro was also moved from the standard library into libcore to have the
  same interface. Otherwise the functions all have copied stability from the
  standard library now.
* The `#![no_std]` attribute
* `fs::DirBuilder`
* `fs::DirBuilder::new`
* `fs::DirBuilder::recursive`
* `fs::DirBuilder::create`
* `os::unix::fs::DirBuilderExt`
* `os::unix::fs::DirBuilderExt::mode`
* `vec::Drain`
* `vec::Vec::drain`
* `string::Drain`
* `string::String::drain`
* `vec_deque::Drain`
* `vec_deque::VecDeque::drain`
* `collections::hash_map::Drain`
* `collections::hash_map::HashMap::drain`
* `collections::hash_set::Drain`
* `collections::hash_set::HashSet::drain`
* `collections::binary_heap::Drain`
* `collections::binary_heap::BinaryHeap::drain`
* `Vec::extend_from_slice` (renamed from `push_all`)
* `Mutex::get_mut`
* `Mutex::into_inner`
* `RwLock::get_mut`
* `RwLock::into_inner`
* `Iterator::min_by_key` (renamed from `min_by`)
* `Iterator::max_by_key` (renamed from `max_by`)

Deprecated APIs

* `ErrorKind::UnexpectedEOF` (renamed to `UnexpectedEof`)
* `OsString::from_bytes`
* `OsStr::to_cstring`
* `OsStr::to_bytes`
* `fs::walk_dir` and `fs::WalkDir`
* `path::Components::peek`
* `slice::bytes::MutableByteVector`
* `slice::bytes::copy_memory`
* `Vec::push_all` (renamed to `extend_from_slice`)
* `Duration::span`
* `IpAddr`
* `SocketAddr::ip`
* `Read::tee`
* `io::Tee`
* `Write::broadcast`
* `io::Broadcast`
* `Iterator::min_by` (renamed to `min_by_key`)
* `Iterator::max_by` (renamed to `max_by_key`)
* `net::lookup_addr`

New APIs (still unstable)

* `<[T]>::sort_by_key` (added to mirror `min_by_key`)

Closes #27585
Closes #27704
Closes #27707
Closes #27710
Closes #27711
Closes #27727
Closes #27740
Closes #27744
Closes #27799
Closes #27801
cc #27801 (doesn't close as `Chars` is still unstable)
Closes #28968

There wasn't any particular reason the functions needed to be there
anyway, so just get rid of them, and adjust libstd to compensate.

With this change, libcore depends on exactly two floating-point functions:
fmod and fmodf.  They are implicitly referenced because they are
used to implement "%".

This commit removes all unstable and deprecated functions in the standard
library. A release was recently cut (1.3) which makes this a good time for some
spring cleaning of the deprecated functions.

This commit is an implementation of [RFC 1184][rfc] which tweaks the behavior of
the `#![no_std]` attribute and adds a new `#![no_core]` attribute. The
`#![no_std]` attribute now injects `extern crate core` at the top of the crate
as well as the libcore prelude into all modules (in the same manner as the
standard library's prelude). The `#![no_core]` attribute disables both std and
core injection.

[rfc]: https://github.com/rust-lang/rfcs/pull/1184

Having the primitive and module docs derived from the same source
causes problems, primarily that they can't contain hyperlinks
cross-referencing each other.

This crates dedicated private modules in `std` to document the
primitive types, then for all primitives that have a corresponding
module, puts hyperlinks in moth the primitive docs and the module docs
cross-linking each other.

This should help clear up confusion when readers find themselves on
the wrong page.

It turns out that the 32-bit toolchain for MSVC has many of these functions as
`static inline` functions in header files so there's not actually a symbol for
Rust to call. All of the implementations just cast floats to their 64-bit
variants and then cast back to 32-bit at the end, so the standard library now
takes this strategy.

These constants have been unstable for some time now already

This commit shards the broad `core` feature of the libcore library into finer
grained features. This split groups together similar APIs and enables tracking
each API separately, giving a better sense of where each feature is within the
stabilization process.

A few minor APIs were deprecated along the way:

* Iterator::reverse_in_place
* marker::NoCopy

This API was exercised in a few tests and mirrors the `from_str_radix`
functionality of the integer types.

This commit removes all the old casting/generic traits from `std::num` that are
no longer in use by the standard library. This additionally removes the old
`strconv` module which has not seen much use in quite a long time. All generic
functionality has been supplanted with traits in the `num` crate and the
`strconv` module is supplanted with the [rust-strconv crate][rust-strconv].

[rust-strconv]: https://github.com/lifthrasiir/rust-strconv

This is a breaking change due to the removal of these deprecated crates, and the
alternative crates are listed above.

[breaking-change]

This commit stabilizes the `std::num` module:

* The `Int` and `Float` traits are deprecated in favor of (1) the
  newly-added inherent methods and (2) the generic traits available in
  rust-lang/num.

* The `Zero` and `One` traits are reintroduced in `std::num`, which
  together with various other traits allow you to recover the most
  common forms of generic programming.

* The `FromStrRadix` trait, and associated free function, is deprecated
  in favor of inherent implementations.

* A wide range of methods and constants for both integers and floating
  point numbers are now `#[stable]`, having been adjusted for integer
  guidelines.

* `is_positive` and `is_negative` are renamed to `is_sign_positive` and
  `is_sign_negative`, in order to address #22985

* The `Wrapping` type is moved to `std::num` and stabilized;
  `WrappingOps` is deprecated in favor of inherent methods on the
  integer types, and direct implementation of operations on
  `Wrapping<X>` for each concrete integer type `X`.

Closes #22985
Closes #21069

[breaking-change]

Now that support has been removed, all lingering use cases are renamed.

* count_ones/zeros, trailing_ones/zeros return u32, not usize
* rotate_left/right take u32, not usize
* RADIX, MANTISSA_DIGITS, DIGITS, BITS, BYTES are u32, not usize

Doesn't touch pow because there's another PR for it.

[breaking-change]

In `std::f32` and `std::f64`:

- `MIN_VALUE` → `MIN`
- `MAX_VALUE` → `MAX`
- `MIN_POS_VALUE` → `MIN_POSITIVE`

This matches the corresponding integer constants.

[breaking-change]

Conflicts:
	src/libcore/atomic.rs
	src/libcore/finally.rs
	src/test/auxiliary/inherited_stability.rs
	src/test/auxiliary/lint_stability.rs

* `core` - for the core crate
* `hash` - hashing
* `io` - io
* `path` - path
* `alloc` - alloc crate
* `rand` - rand crate
* `collections` - collections crate
* `std_misc` - other parts of std
* `test` - test crate
* `rustc_private` - everything else