Auto merge of rust-lang#95555 - nnethercote:parse_tt-new-representation, r=petrochenkov

A new matcher representation for use in `parse_tt`

By transforming the matcher into a different form, `parse_tt` can run faster and be easier to understand.

r? `@petrochenkov`

Author: bors

compiler/rustc_expand/src/lib.rs

@@ -1,7 +1,5 @@
 #![feature(associated_type_bounds)]
 #![feature(associated_type_defaults)]
-#![feature(box_patterns)]
-#![feature(box_syntax)]
 #![feature(crate_visibility_modifier)]
 #![feature(decl_macro)]
 #![feature(if_let_guard)]