Rollup merge of #100291 - WaffleLapkin:cstr_const_methods, r=oli-obk

constify some `CStr` methods

This PR marks the following public APIs as `const`:
```rust
impl CStr {
    // feature(const_cstr_from_bytes)
    pub const fn from_bytes_until_nul(bytes: &[u8]) -> Result<&CStr, FromBytesUntilNulError>;
    pub const fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, FromBytesWithNulError>;

    // feature(const_cstr_to_bytes)
    pub const fn to_bytes(&self) -> &[u8];
    pub const fn to_bytes_with_nul(&self) -> &[u8];
    pub const fn to_str(&self) -> Result<&str, str::Utf8Error>;
}
```

r? ```@oli-obk``` (use of `const_eval_select` :P )
cc ```@mina86``` (you've asked for this <3 )

library/core/src/lib.rs

@@ -159,6 +159,7 @@
 #![feature(const_slice_from_ref)]
 #![feature(const_slice_index)]
 #![feature(const_is_char_boundary)]
+#![feature(const_cstr_methods)]
 //
 // Language features:
 #![feature(abi_unadjusted)]