Stabilize `core::hint::cold_path`
`cold_path` has been around unstably for a while and is a rather useful tool to have. It does what it is supposed to and there are no known remaining issues, so stabilize it here (including const).
Newly stable API:
```rust
// in core::hint
pub const fn cold_path();
```
I have opted to exclude `likely` and `unlikely` for now since they have had some concerns about ease of use that `cold_path` doesn't suffer from. `cold_path` is also significantly more flexible; in addition to working with boolean `if` conditions, it can be used in `match` arms, `if let`, closures, and other control flow blocks. `likely` and `unlikely` are also possible to implement in user code via `cold_path`, if desired.
Closes: https://github.com/rust-lang/rust/issues/136873 (tracking issue)
---
There has been some design and implementation work for making `#[cold]` function in more places, such as `if` arms, `match` arms, and closure bodies. Considering a stable `cold_path` will cover all of these usecases, it does not seem worth pursuing a more powerful `#[cold]` as an alternative way to do the same thing. If the lang team agrees, then:
Closes: https://github.com/rust-lang/rust/issues/26179
Closes: https://github.com/rust-lang/rust/pull/120193
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