Add char::code_point

[Aehmlo]

In #79502, which introduced From<char> for u{64,128}, some desire for a dedicated code point method on char was expressed. This PR implements char::code_point (where the underscore comes from the fact that the documentation and Unicode both stylize it as two words, i.e., "code point" instead of "codepoint"). The method is simply implemented as self as u32.

[rust-highfive]

r? @dtolnay

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[Aehmlo]

I see that CI failed due to a missing stability attribute. I admit to some confusion about how to handle this: does adding the attribute to library/core/src/char/methods.rs suffice, or do I also need to register the feature in compiler/rustc_feature?

[crlf0710]

@Aehmlo For library functions the former will suffice.

[Aehmlo]

All right, I think this one should do it.

[joshtriplett]

https://doc.rust-lang.org/core/str/fn.next_code_point.html serves as further naming precedent.

[joshtriplett]

This should not be insta-stable. It should be marked as unstable, with a tracking issue and a feature gate.

Otherwise the method itself seems reasonable to me.

[JohnCSimon]

ping from triage:
@Aehmlo - can you address the comments from joshtriplett?

[camelid]

Docs for the #[unstable] attribute: https://rustc-dev-guide.rust-lang.org/stability.html#unstable

[Aehmlo]

Yes, I'll stick this behind a feature gate (and probably rebase) when I get a chance — hopefully this weekend. Thanks for the link to the docs!

[crlf0710]

@Aehmlo any updates on this?

[JohnCSimon]

triage:
@Aehmlo I'm closing this as inactive, please feel free to reopen when you want to continue with this.
Thank you!