Make ascii methods on u8 insta-stable
Make ascii methods on u8 insta-stable

Those methods will shadow the methods of `AsciiExt`, so if we don't
make them insta-stable, everyone will hitting stability errors. It
is fine adding those as stable, because they are just being moved
around [according to sfackler][1].

OPEN QUESTION: this commit also stabilizes the `AsciiExt` methods
that were previously feature gated by the `ascii_ctype` feature.
Maybe we don't want to stablilize those yet.

[1]: https://github.com/rust-lang/rust/pull/44042#issuecomment-329939279

aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust