`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:
// 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.
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