Simplify `TokenTree` and fix `macro_rules!` bugs
This PR
- fixes #39390, fixes #39403, and fixes #39404 (each is a [breaking-change], see issues for examples),
- fixes #39889,
- simplifies and optimizes macro invocation parsing,
- cleans up `ext::tt::transcribe`,
- removes `tokenstream::TokenTree::Sequence` and `Token::MatchNt`,
- instead, adds a new type `ext::tt::quoted::TokenTree` for use by `macro_rules!` (`ext::tt`)
- removes `parser.quote_depth` and `parser.parsing_token_tree`, and
- removes `quote_matcher!`.
- Instead, use `quote_tokens!` and `ext::tt::quoted::parse` the result with `expect_matchers=true`.
- I found no outside uses of `quote_matcher!` when searching Rust code on Github.
r? @nrc