cf6f64a963
This commit makes the following functions from `core::str` `const fn`: - `from_utf8[_mut]` (`feature(const_str_from_utf8)`) - `from_utf8_unchecked_mut` (`feature(const_str_from_utf8_unchecked_mut)`) - `Utf8Error::{valid_up_to,error_len}` (`feature(const_str_from_utf8)`)