Generalize type signature of usingStateT

[ericpashman]

This PR changes two characters to generalize the type of usingStateT, making it possible to use it with functions that arbitrarily transform the items in a stream.

Here's an example motivating the need for this change. I rely on the below function to transform streams in a manner in which each item in a stream can be mapped to a new value 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 the present type of usingStateT overly constrains the type of the argument 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.)

[harendra-kumar]

LGTM. Thanks for the PR.

There is no prepackaged functionality for this but scanMaybe may be relevant for your use case.