tokio-1.18.0

# 1.18.0 (April 27, 2022)

This release adds a number of new APIs in `tokio::net`, `tokio::signal`, and
`tokio::sync`. In addition, it adds new unstable APIs to `tokio::task` (`Id`s
for uniquely identifying a task, and `AbortHandle` for remotely cancelling a
task), as well as a number of bugfixes.

### Fixed

- blocking: add missing `#[track_caller]` for `spawn_blocking` (#4616)
- macros: fix `select` macro to process 64 branches (#4519)
- net: fix `try_io` methods not calling Mio's `try_io` internally (#4582)
- runtime: recover when OS fails to spawn a new thread (#4485)

### Added

- net: add `UdpSocket::peer_addr` (#4611)
- net: add `try_read_buf` method for named pipes (#4626)
- signal: add `SignalKind` `Hash`/`Eq` impls and `c_int` conversion (#4540)
- signal: add support for signals up to `SIGRTMAX` (#4555)
- sync: add `watch::Sender::send_modify` method (#4310)
- sync: add `broadcast::Receiver::len` method (#4542)
- sync: add `watch::Receiver::same_channel` method (#4581)
- sync: implement `Clone` for `RecvError` types (#4560)

### Changed

- update `mio` to 0.8.1 (#4582)
- macros: rename `tokio::select!`'s internal `util` module (#4543)
- runtime: use `Vec::with_capacity` when building runtime (#4553)

### Documented

- improve docs for `tokio_unstable` (#4524)
- runtime: include more documentation for thread_pool/worker (#4511)
- runtime: update `Handle::current`'s docs to mention `EnterGuard` (#4567)
- time: clarify platform specific timer resolution (#4474)
- signal: document that `Signal::recv` is cancel-safe (#4634)
- sync: `UnboundedReceiver` close docs (#4548)

### Unstable

The following changes only apply when building with `--cfg tokio_unstable`:

- task: add `task::Id` type (#4630)
- task: add `AbortHandle` type for cancelling tasks in a `JoinSet` (#4530],
  [#4640)
- task: fix missing `doc(cfg(...))` attributes for `JoinSet` (#4531)
- task: fix broken link in `AbortHandle` RustDoc (#4545)
- metrics: add initial IO driver metrics (#4507)