Auto merge of #111524 - scottmcm:escape-using-ascii, r=cuviper

`ascii::Char`-ify the escaping code in `core`

This means that `EscapeIterInner::as_str` no longer needs unsafe code, because the type system ensures the internal buffer is only ASCII, and thus valid UTF-8.

Come to think of it, this also gives it a (non-guaranteed) niche.

cc `@BurntSushi` as potentially interested
`ascii::Char` tracking issue: #110998

library/core/src/lib.rs

@@ -216,6 +216,7 @@
 #![feature(intra_doc_pointers)]
 #![feature(intrinsics)]
 #![feature(lang_items)]
+#![feature(let_chains)]
 #![feature(link_llvm_intrinsics)]
 #![feature(macro_metavar_expr)]
 #![feature(min_specialization)]