Build a custom CFG parser to parse rule files

[zyxw59]

Currently, the plan is to use LALRPOP to generate a parser for the rule file grammar. However, I don't know how easy it is to use LALRPOP with the custom tokenizer in the token module. Additionally, LALRPOP itself takes a long time to compile and has a lot of requirements, so it would be nice to cut it out.

[zyxw59]

Looking over the grammar in efd0ae1, I think a CFG is unnecessary for most of the grammar. Places where I think a CFG is necessary: the match and environment of rules, which can be arbitrary regular expressions, and the generalized environment syntax, which can contain arbitrarily-nested parentheses.

I propose matching the elements of a rule file using a more lenient regular expression, and then using a much simpler CFG parser on those parts of the matches that require it.

[zyxw59]

Reopening since I still need to write a parser for category definitions.