Stabilize `ops::ControlFlow` (just the type)

Tracking issue: https://github.com/rust-lang/rust/issues/75744 (which also tracks items *not* closed by this PR).

With the new `?` desugar implemented, [it's no longer possible to mix `Result` and `ControlFlow`](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=13feec97f5c96a9d791d97f7de2d49a6).  (At the time of making this PR, godbolt was still on the 2021-05-01 nightly, where you can see that [the mixing example compiled](https://rust.godbolt.org/z/13Ke54j16).)  That resolves the only blocker I know of, so I'd like to propose that `ControlFlow` be considered for stabilization.

Its basic existence was part of https://github.com/rust-lang/rfcs/pull/3058, where it got a bunch of positive comments (examples [1](https://github.com/rust-lang/rfcs/pull/3058#issuecomment-758277325) [2](https://github.com/rust-lang/rfcs/pull/3058#pullrequestreview-592106494) [3](https://github.com/rust-lang/rfcs/pull/3058#issuecomment-784444155) [4](https://github.com/rust-lang/rfcs/pull/3058#issuecomment-797031584)).  Its use in the compiler has been well received (https://github.com/rust-lang/rust/pull/78182#issuecomment-713695594), and there are ecosystem updates interested in using it (https://github.com/rust-itertools/itertools/issues/469#issuecomment-677729589, https://github.com/jonhoo/rust-imap/issues/194).

As this will need an FCP, picking a libs member manually:
r? `@m-ou-se`

## Stabilized APIs

```rust
#[derive(Debug, Clone, Copy, PartialEq)]
pub enum ControlFlow<B, C = ()> {
    /// Exit the operation without running subsequent phases.
    Break(B),
    /// Move on to the next phase of the operation as normal.
    Continue(C),
}
```

As well as using `?` on a `ControlFlow<B, _>` in a function returning `ControlFlow<B, _>`.  (Note, in particular, that there's no `From::from`-conversion on the `Break` value, the way there is for `Err`s.)

## Existing APIs *not* stabilized here

All the associated methods and constants: `break_value`, `is_continue`, `map_break`, [`CONTINUE`](https://doc.rust-lang.org/nightly/std/ops/enum.ControlFlow.html#associatedconstant.CONTINUE), etc.

Some of the existing methods in nightly seem reasonable, some seem like they should be removed, and some need more discussion to decide.  But none of them are *essential*, so [as in the RFC](https://rust-lang.github.io/rfcs/3058-try-trait-v2.html#methods-on-controlflow), they're all omitted from this PR.

They can be considered separately later, as further usage demonstrates which are important.