RFC: std::ascii reform

Move the `std::ascii::Ascii` type and related traits to a new Cargo package on crates.io, and instead expose its functionality for `u8`, `[u8]`, `char`, and `str` types.

[Rendered](https://github.com/SimonSapin/rfcs/blob/std-ascii-reform/text/0000-std-ascii-reform.md)