Move std::ascii into core and/or collections

[kmcallister]

I'd like to use it from a #![no_std] crate.

[alexcrichton]

cc #16355

The most relevant part is @aturon's concerns here: #16355 (comment).

At this time we are not sure that we'd like to do this.

[aturon]

@kmcallister I'd be interested to get a bit more detail on the specific functionality you're wanting to use from the ascii module.

[kmcallister]

Okay; you can see here which parts of ascii.rs I copied in to my project. I basically need case conversion and case-insensitive comparison, as well as basic character properties (alphanum, lower case).

The HTML syntax spec is written in terms of ASCII case, and I want my implementation to be usable from C, so I'd rather not link libstd.

[alexcrichton]

I think that some of the more useful aspects of std::ascii relate to how it operates over vectors and strings, not just chars and slices, so I would suspect that may belong more in collections than core, but it may want to still be scrutinized beforehand.

[frewsxcv]

There is a third party crate for ascii available here:

https://github.com/SimonSapin/rust-ascii

This can probably be closed

[steveklabnik]

agreed!