This commit is an implementation of [RFC 503][rfc] which is a stabilization
story for the prelude. Most of the RFC was directly applied, removing reexports.
Some reexports are kept around, however:

* `range` remains until range syntax has landed to reduce churn.
* `Path` and `GenericPath` remain until path reform lands. This is done to
  prevent many imports of `GenericPath` which will soon be removed.
* All `io` traits remain until I/O reform lands so imports can be rewritten all
  at once to `std::io::prelude::*`.

This is a breaking change because many prelude reexports have been removed, and
the RFC can be consulted for the exact list of removed reexports, as well as to
find the locations of where to import them.

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0503-prelude-stabilization.md
[breaking-change]

Closes #20068

This commit is an implementation of [RFC 526][rfc] which is a change to alter
the definition of the old `fmt::FormatWriter`. The new trait, renamed to
`Writer`, now only exposes one method `write_str` in order to guarantee that all
implementations of the formatting traits can only produce valid Unicode.

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0526-fmt-text-writer.md

One of the primary improvements of this patch is the performance of the
`.to_string()` method by avoiding an almost-always redundant UTF-8 check. This
is a breaking change due to the renaming of the trait as well as the loss of the
`write` method, but migration paths should be relatively easy:

* All usage of `write` should move to `write_str`. If truly binary data was
  being written in an implementation of `Show`, then it will need to use a
  different trait or an altogether different code path.

* All usage of `write!` should continue to work as-is with no modifications.

* All usage of `Show` where implementations just delegate to another should
  continue to work as-is.

[breaking-change]

Closes #20352

This pass performs a second pass of stabilization through the `std::sync`
module, avoiding modules/types that are being handled in other PRs (e.g.
mutexes, rwlocks, condvars, and channels).

The following items are now stable

* `sync::atomic`
* `sync::atomic::ATOMIC_BOOL_INIT` (was `INIT_ATOMIC_BOOL`)
* `sync::atomic::ATOMIC_INT_INIT` (was `INIT_ATOMIC_INT`)
* `sync::atomic::ATOMIC_UINT_INIT` (was `INIT_ATOMIC_UINT`)
* `sync::Once`
* `sync::ONCE_INIT`
* `sync::Once::call_once` (was `doit`)
  * C == `pthread_once(..)`
  * Boost == `call_once(..)`
  * Windows == `InitOnceExecuteOnce`
* `sync::Barrier`
* `sync::Barrier::new`
* `sync::Barrier::wait` (now returns a `bool`)
* `sync::Semaphore::new`
* `sync::Semaphore::acquire`
* `sync::Semaphore::release`

The following items remain unstable

* `sync::SemaphoreGuard`
* `sync::Semaphore::access` - it's unclear how this relates to the poisoning
                              story of mutexes.
* `sync::TaskPool` - the semantics of a failing task and whether a thread is
                     re-attached to a thread pool are somewhat unclear, and the
                     utility of this type in `sync` is question with respect to
                     the jobs of other primitives. This type will likely become
                     stable or move out of the standard library over time.
* `sync::Future` - futures as-is have yet to be deeply re-evaluated with the
                   recent core changes to Rust's synchronization story, and will
                   likely become stable in the future but are unstable until
                   that time comes.

[breaking-change]

Part of #19253

[breaking-change]

This patch marks `PartialEq`, `Eq`, `PartialOrd`, and `Ord` as
`#[stable]`, as well as the majorify of manual implementaitons of these
traits. The traits match the [reform
RFC](https://github.com/rust-lang/rfcs/pull/439).

Along the way, two changes are made:

* The recently-added type parameters for `Ord` and `Eq` are
  removed. These were mistakenly added while adding them to `PartialOrd`
  and `PartialEq`, but they don't make sense given the laws that are
  required for (and use cases for) `Ord` and `Eq`.

* More explicit laws are added for `PartialEq` and `PartialOrd`,
  connecting them to their associated mathematical concepts.

In the future, many of the impls should be generalized; see
since generalizing later is not a breaking change.

[breaking-change]

This commit takes a second pass through the `slice` module to
stabilize its API. The changes are as follows:

**Stable**:

* `as_mut_slice`
* `as_ptr`, `as_mut_ptr`
* `binary_search_by` (was: `binary_search`)
* `binary_search` (was: `binary_search_elem`)
* `chunks`, `chunks_mut`
* `contains`
* `ends_with`
* `first`, `first_mut` (was: `head`)
* `get_unchecked`, `get_unchecked_mut` (was: `unsafe_get`)
* `get`
* `is_empty`
* `iter`, `iter_mut`
* `len`
* `reverse`
* `sort_by`
* `sort`
* `split_at`, `split_at_mut`
* `split_mut`, `splitn_mut`, `rsplitn_mut`
* `split`, `splitn`, `rsplitn`
* `starts_with`
* `swap`
* `to_vec`
* `windows`

**Deprecated**:

* `head`, `head_mut` (renamed as above)
* `unsafe_get`, `unsafe_mut` (renamed as above)
* `binary_search_elem` (renamed as above)
* `partitioned`, deprecated in favor of a new, more
  general iterator consumer called `partition`.
* `BinarySearchResult`, deprecated in favor of `Result<uint, uint>`

[breaking-change]

Part of #19253

[breaking-change]

It's useful to be able to save state.

[breaking-change]

The `mut` in slices is now redundant. Mutability is 'inferred' from position. This means that if mutability is only obvious from the type, you will need to use explicit calls to the slicing methods.

Note that this doesn't add the surface syntax.

This commit performs a second pass for stabilization over the `std::ptr` module.
The specific actions taken were:

* The `RawPtr` trait was renamed to `PtrExt`
* The `RawMutPtr` trait was renamed to `MutPtrExt`
* The module name `ptr` is now stable.
* These functions were all marked `#[stable]` with no modification:
  * `null`
  * `null_mut`
  * `swap`
  * `replace`
  * `read`
  * `write`
  * `PtrExt::is_null`
  * `PtrExt::offset`
* These functions remain unstable:
  * `as_ref`, `as_mut` - the return value of an `Option` is not fully expressive
                         as null isn't the only bad value, and it's unclear
                         whether we want to commit to these functions at this
                         time. The reference/lifetime semantics as written are
                         also problematic in how they encourage arbitrary
                         lifetimes.
  * `zero_memory` - This function is currently not used at all in the
                    distribution, and in general it plays a broader role in the
                    "working with unsafe pointers" story. This story is not yet
                    fully developed, so at this time the function remains
                    unstable for now.
  * `read_and_zero` - This function remains unstable for largely the same
                      reasons as `zero_memory`.
* These functions are now all deprecated:
  * `PtrExt::null` - call `ptr::null` or `ptr::null_mut` instead.
  * `PtrExt::to_uint` - use an `as` expression instead.
  * `PtrExt::is_not_null` - use `!p.is_null()` instead.

Also get rid of NoSync markers since UnsafeCell is now not Sync

This commit is an implementation of [RFC 503][rfc] which is a stabilization
story for the prelude. Most of the RFC was directly applied, removing reexports.
Some reexports are kept around, however:

* `range` remains until range syntax has landed to reduce churn.
* `Path` and `GenericPath` remain until path reform lands. This is done to
  prevent many imports of `GenericPath` which will soon be removed.
* All `io` traits remain until I/O reform lands so imports can be rewritten all
  at once to `std::io::prelude::*`.

This is a breaking change because many prelude reexports have been removed, and
the RFC can be consulted for the exact list of removed reexports, as well as to
find the locations of where to import them.

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0503-prelude-stabilization.md
[breaking-change]

Closes #20068

Mostly following the convention in RFC 356

RacyCell is not exactly what we'd like as a final implementation for
this. Therefore, we're moving it under `std::comm` and also making it
private.

Made iterator-returning methods return newtypes
Adjusted some docs to be forwards compatible with a generic pattern API