Stabilize const_cstr_methods

[tgross35]

This PR seeks to stabilize const_cstr_methods. Fixes most of #101719

New const stable API

impl CStr {
    // depends: memchr
    pub const fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, FromBytesWithNulError> {...}
    // depends: const_slice_index
    pub const fn to_bytes(&self) -> &[u8] {}
    // depends: pointer casts
    pub const fn to_bytes_with_nul(&self) -> &[u8] {}
    // depends: str::from_utf8
    pub const fn to_str(&self) -> Result<&str, str::Utf8Error> {}
}

I don't think any of these methods will have any issue when CStr becomes a thin pointer as long as memchr is const (which also allows for const strlen) .

Notes

• from_bytes_until_nul relies on const_slice_index, which relies on const_trait_impls, and generally this should be avoided. After talking with Oli, it should be OK in this case because we could replace the ranges with pointer tricks if needed (worst case being those feature gates disappear). Stabilize const_cstr_methods #107624 (comment)
• Making from_ptr const is deferred because it depends on const_eval_select. I have moved this under the new flag const_cstr_from_ptr Stabilize const_cstr_methods #107624 (comment)

cc @oli-obk I think you're the const expert

@rustbot modify labels: +T-libs-api +needs-fcp