77c985f765
Make slice->str conversion and related functions `const`
This PR marks the following APIs as `const`:
```rust
// core::str
pub const fn from_utf8(v: &[u8]) -> Result<&str, Utf8Error>;
pub const fn from_utf8_mut(v: &mut [u8]) -> Result<&mut str, Utf8Error>;
pub const unsafe fn from_utf8_unchecked_mut(v: &mut [u8]) -> &mut str;
impl Utf8Error {
pub const fn valid_up_to(&self) -> usize;
pub const fn error_len(&self) -> Option<usize>;
}
```
Everything but `from_utf8_unchecked_mut` uses `const_str_from_utf8` feature gate, `from_utf8_unchecked_mut` uses `const_str_from_utf8_unchecked_mut` feature gate.
---
I'm not sure why `from_utf8_unchecked_mut` was left out being non-`const`, considering that `from_utf8_unchecked` is not only `const`, but **`const` stable**.
---
r? ```@oli-obk``` (performance-only `const_eval_select` use)
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