Stabilize integer logarithms Stabilizes feature `int_log`. I've also made the functions const stable, because they don't depend on any unstable const features. `rustc_allow_const_fn_unstable` is just there for `Option::expect`, which could be replaced with a `match` and `panic!`. cc ``@rust-lang/wg-const-eval`` closes https://github.com/rust-lang/rust/issues/70887 (tracking issue) ~~blocked on FCP finishing: https://github.com/rust-lang/rust/issues/70887#issuecomment-1289028216~~ FCP finished: https://github.com/rust-lang/rust/issues/70887#issuecomment-1302121266