Generalize type signature of usingStateT

[ericpashman]

This PR changes two characters to generalize the type of usingStateT. This permits its use with functions that arbitrarily transform the items in a stream.

Here is a motiving example. I rely on the below function as a way to write stream transformations in which each item in a stream can be transformed in a way that depends on an arbitrary state (which is typically built up from previous items in the stream):

import qualified Streamly.Data.Stream.Prelude  as S
import qualified Streamly.Internal.Data.Stream as S

stateMap
  :: Monad m => (a -> StateT s m (Maybe b)) -> s -> Stream m a -> Stream m b
stateMap f s = S.catMaybes . S.evalStateT (return s) . S.mapM f . S.liftInner

It should be possible to refactor this function as stateMap f s = S.catMaybes . S.usingStateT (return s) (S.mapM f), but presently the type of usingStateT would overly constrain the type of f.

(I'll also take this opportunity to ask: Is there is an established approach to doing the sort of thing I wrote stateMap for? I couldn't find any such pre-packaged functionality in the library.)

[ericpashman]

Whoops, I just realized I made the change to the deprecated version of this function. Closing this: I'll make a new PR for the equivalent change in the right place.