Levity polymorphism

Currently we can only return LiftedRep types from parsers. Sometimes this performs poorly, when we want to parse Int, Double, or other directly unboxable types. While sometimes it's possible to avoid boxing by using tail-recursive functions with unboxed accumulators, it's generally more convenient (and possibly more efficient) if we can use non-tail recursion and basic combinators with unboxed return values.

Unfortunately the ergonomics of levity polymorphism is awful. Example: https://hackage.haskell.org/package/bytesmith-0.3.7.0/docs/Data-Bytes-Parser-Rebindable.html I think something like this would be acceptable, but for the sake of usability more functions should be included in levity-polymorphic classes. This would require boilerplate (or code generation) on the implementation side, but would be hopefully more convenient on user's side.

Start by reviewing the levity-polymorphic API described in the linked Data.Bytes.Parser.Rebindable example and identify which parser functions and classes need broader coverage. Define the desired user-facing API and implementation approach before making changes; done means parsers can return directly unboxable types such as Int and Double without the current LiftedRep limitation.