Especially when documenting the use of `0`, since zero looks very
similar to `O` in fonts not meant for displaying code.

Other literal characters, traits, etc should also use code formatting.
This change makes this documentation more internally consistent.

This is now std::io::Result

Fixes #24767

Fixes #24656

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

It's just as convenient, but it's much faster. Using write! requires an
extra call to fmt::write and a extra dynamically dispatched call to
Arguments' Display format function.

This commit removes the reexports of `old_io` traits as well as `old_path` types
and traits from the prelude. This functionality is now all deprecated and needs
to be removed to make way for other functionality like `Seek` in the `std::io`
module (currently reexported as `NewSeek` in the io prelude).

Closes #23377
Closes #23378

Fixes #22927

This brings comments in line with https://github.com/rust-lang/rfcs/blob/master/text/0505-api-comment-conventions.md#using-markdown

Removes deprecated `{:08d}` format from the module documentation.
`{:08}` should be used instead now.

Toss the tomatoes!

This brings it in line with its namesake in `std::io`.

[breaking-change]

There are a number of holes that the stability lint did not previously cover,
including:

* Types
* Bounds on type parameters on functions and impls
* Where clauses
* Imports
* Patterns (structs and enums)

These holes have all been fixed by overriding the `visit_path` function on the
AST visitor instead of a few specialized cases. This change also necessitated a
few stability changes:

* The `collections::fmt` module is now stable (it was already supposed to be).
* The `thread_local::imp::Key` type is now stable (it was already supposed to
  be).
* The `std::rt::{begin_unwind, begin_unwind_fmt}` functions are now stable.
  These are required via the `panic!` macro.
* The `std::old_io::stdio::{println, println_args}` functions are now stable.
  These are required by the `print!` and `println!` macros.
* The `ops::{FnOnce, FnMut, Fn}` traits are now `#[stable]`. This is required to
  make bounds with these traits stable. Note that manual implementations of
  these traits are still gated by default, this stability only allows bounds
  such as `F: FnOnce()`.

Additionally, the compiler now has special logic to ignore its own generated
`__test` module for the `--test` harness in terms of stability.

Closes #8962
Closes #16360
Closes #20327

[breaking-change]

Fixes #16803.
Fixes #14342.
Fixes half of #21827 -- slice syntax is still broken.