Consider converting GRAMMAR.md into test

I'd like to convert the reference grammar in GRAMMAR.md into something that can actually be parsed by an LR(k) parser-generator, so that the grammar can be checked for ambiguity.

I like the hand-written parser, so I'm not saying that Just should actually replace it's parser with a generated one, just that we should add a test that ensure that the language stays LR(k) using the grammar.

I think the best choice here is probably https://github.com/lalrpop/lalrpop.