Provide grouping operations for conduit using the FreeT trick from pipes-group.

[danidiaz]

Right now, the grouping-related functions of streamy-sig are not implemented in streamy-conduit.

There doesn't seem to be a "conduit-group" package analoguous to pipes-group which provides grouping operations that preserve streaming and avoid having to hold entire groups in memory at any given time.

However, C.ConduitM () is a functor and, if I'm not mistaken, the FreeT-based approach used by pipes-group could work with it, too.