Refactor parser

[BinderDavid]

• Bump lalrpop to 0.20 which is up to 2x faster in compilation of grammars.
• Make the lalrpop grammar more readable by factoring out individual expressions and toplevel declarations.
• Make all the enum variants of cst::Exp into their own respective struct. This already simplifies the implementation of lowering, and enables even more simplifications once we can do the same thing for the ust/tst. (I don't want to do the same split there while we still have all the generic functions.)
• ParamInst and TelescopeInst are not relevant for the CST, so I removed them there.