Auto merge of #130738 - bjoernager:const-make-ascii, r=jhpratt

Mark `make_ascii_uppercase` and `make_ascii_lowercase` in `[u8]` and `str` as const.

Relevant tracking issue: #130698

This PR extends #130697 and #130713 to the similar methods in byte slices (`[u8]`) and string slices (`str`).

For the `str` methods, this simply requires adding the `const` specifier to the function signatures. The `[u8]` methods, however, require (at least a temporary) reimplementation due to the use of iterators and `for` loops.

library/core/src/lib.rs

@@ -132,6 +132,7 @@
 #![feature(const_ipv4)]
 #![feature(const_ipv6)]
 #![feature(const_likely)]
+#![feature(const_make_ascii)]
 #![feature(const_maybe_uninit_assume_init)]
 #![feature(const_nonnull_new)]
 #![feature(const_num_midpoint)]
@@ -150,6 +151,7 @@
 #![feature(const_slice_from_raw_parts_mut)]
 #![feature(const_slice_from_ref)]
 #![feature(const_slice_split_at_mut)]
+#![feature(const_str_as_mut)]
 #![feature(const_str_from_utf8_unchecked_mut)]
 #![feature(const_strict_overflow_ops)]
 #![feature(const_swap)]