Constify [u8]::is_ascii (unstably)

UTF-8 checking in `const fn`-stabilized back in 1.63, but apparently somehow ASCII checking was never const-ified, despite being simpler.

library/core/src/lib.rs

@@ -149,6 +149,7 @@
 #![feature(const_slice_from_raw_parts_mut)]
 #![feature(const_slice_from_ref)]
 #![feature(const_slice_index)]
+#![feature(const_slice_is_ascii)]
 #![feature(const_slice_ptr_len)]
 #![feature(const_slice_split_at_mut)]
 #![feature(const_str_from_utf8_unchecked_mut)]