Mark and implement 'make_ascii_uppercase' and 'make_ascii_lowercase' in '[u8]' and 'str' as const;

library/core/src/lib.rs

@@ -133,6 +133,7 @@
 #![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)]
@@ -151,6 +152,7 @@
 #![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)]