Rollup merge of #103570 - lukas-code:stabilize-ilog, r=scottmcm

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 rust-lang/rust#70887 (tracking issue)

~~blocked on FCP finishing: rust-lang/rust#70887 (comment)
FCP finished: rust-lang/rust#70887 (comment)

Author: Dylan-DPC