add `c_str!` macro to create `&'static CStr`

[davidhewitt]

With Rust 1.77 finally stabilising c"" c-string literals, I've wanted for a little while to be able to make better use of &'static CStr inside PyO3.

This PR proposes a pathway to doing that, by adding a c_str!("foo") macro to create a static c-string safely. With MSRV 1.63 we're able to support this on all versions.

This has the nice effect of moving work from runtime to compile time, because at the moment we have a lot of &str which need to be converted to &CStr using runtime checks. In the future when MSRV passes 1.77 we have a trivial cleanup to swap everything to c"" literals.

The only user-facing changes:

• Add pyo3_ffi::c_str macro (also accessible as pyo3::ffi::c_str).
• The PyCFunction constructors now take &'static CStr where they previously took &'static str.

[Icxolu]

Big 👍 for this from me as well! This is a lot nicer and safer than having to remember to add the \0 and it also gets rid of a lot of casting which is nice to see.

[davidhewitt]

Given the two broadly positive sentiments, I think I'm going to merge this once I get CI passing. It'll make #4254 easier to finish implementing, which I want to complete ASAP with a view to then shipping 0.22.

If there are changes which are wanted (like the PythonDoc changes proposed above) let's put them into follow-up PRs or issues to land after we've shipped and unblocked everyone downstream from 3.13 testing.