Adding PrimMonad instance for parsers

What do you think about adding PrimMonad instances for parses? It does add dependency on primitive but I don't think it's too bad. It provides standard API for working with mutable state and all required pieces are already in place.

instance PrimMonad (ParserT (State# s) e) where
  type PrimState (ParserT (State# s) e) = s
  primitive = liftST . primitive
  {-# INLINE primitive #-}

Main reason to add them is to allow passing parsers as callbacks into functions working with mutable state:

replicateVec :: Int -> Parser e a -> Parser e (Vector a)
replicateVec n elt = embedParserST $ do
  mvec <- MV.replicateM n (embedParser elt)
  V.unsafeFreeze mvec

without PrimMonad instance it's not possible to pass parser as parameter to replicateM and one will have to write loop by hand

Start by reviewing the ParserT definition and the existing liftST, embedParserST, and primitive entry points referenced in the issue. Check the primitive dependency and the parser transformer instances, then verify the proposed PrimMonad behavior with the mutable-vector callback example; done means the instance is integrated without breaking existing parser APIs.