Auto merge of #134824 - niklasf:int_from_ascii, r=ibraheemdev

Implement `int_from_ascii` (#134821)

Provides unstable `T::from_ascii()` and `T::from_ascii_radix()` for integer types `T`, as drafted in tracking issue #134821.

To deduplicate documentation without additional macros, implementations of `isize` and `usize` no longer delegate to equivalent integer types. After #132870 they are inlined anyway.

Author: bors