Fast, LALR backend

[alvinwan]

The commits in this PR are all created by Codex -- with some light editing in a few cases.

• However, there are a few very brittle rules that overfit to the tests. Either I ask Codex to refactor, or I start over and curate more careful, instead of blindly accepting Codex patches.
• On one hand, this was a fun exercise to try Codex, but more importantly, this shows just how much faster the LALR backend is. On main, the entire test suite of 123 tests takes about 4s to run. On this branch, 84 tests takes ~0.2s to run. That's ~20x faster.

As is, this PR isn't really ready to merge, but it's a proof of concept -- and a quick Codex-generated version is good enough for a poc.