Auto merge of #95159 - nnethercote:TtParser, r=petrochenkov

Introduce `TtParser`

These commits make a number of changes to declarative macro expansion, resulting in code that is shorter, simpler, and faster.

Best reviewed one commit at a time.

r? `@petrochenkov`

diff --cc compiler/rustc_expand/src/mbe/transcribe.rs
Simple merge