Split up PState
The PState record is huge (25 fields) and will be updated on every token lexed. That's about 25*8=200 bytes allocation for every byte lexed, for PState update alone! I see that we already store a ParserOpts field in PState that will probably be mostly constant throughout. But there are also a lot of other fields which probably don't change that often, which we could push into a secondary state parameter. E.g. maybe we can manage to have a HotPState (buffer, last_tk, *loc, ...) that is small and changes for every AlexToken emitted and a SlowPState (warnings, errors, tab_fist, ...) that changes only sometimes. Maybe split up that one too into a State record for things that are modified together. We could thread them as separate parameters in the P monad or as a tuple/strict PState record.