Rename to_ascii_{lower,upper} to to_ascii_{lower,upper}case, per #14401

[breaking-change]
Remove Ascii, AsciiCast, OwnedAsciiCast, AsciiStr, IntoBytes, IntoString.

As a replacement, add is_ascii() to AsciiExt, and implement AsciiExt for u8 and char.

[breaking-change]
std::ascii: Use u8 methods rather than the maps directly.
Fix up remaining usage of `to_ascii`.
Fix up remaining usage of `to_ascii`.
typeck: boxed closures can't capture by value

closes #19141
closes #20193
closes #20228
Require types to opt-in Sync
Rename `OwnedPtr` to `UniquePtr`
Make Send and Sync traits unsafe
Move RacyCell to `std::comm`

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.
Implement `Sync` for some windows sys types
Rename `UniquePtr` to `Unique`

Mostly following the convention in RFC 356
Relax `Arc` bounds don't require Sync+Send

Besides the above making sense, it'll also allow us to make `RacyCell`
private and use UnsafeCell instead.
Keep track of the whole error chain
Make Barrier and Condvar Sync/Send
Impl Sync/Send for Rawlink
Implement Sync/Send for ArcInner and Weak
Add a Racy type to bench tests
Implement RaceBox for StdinReader
Implement Sync/Send for windows TCP types