ascii::Char-ify the escaping code

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.

library/core/src/lib.rs

@@ -215,6 +215,7 @@
 #![feature(intra_doc_pointers)]
 #![feature(intrinsics)]
 #![feature(lang_items)]
+#![feature(let_chains)]
 #![feature(link_llvm_intrinsics)]
 #![feature(macro_metavar_expr)]
 #![feature(min_specialization)]