Add `ManagedT`

[Yo Eight]

Could allow an alternative definition of runMVC like this

runMVCT :: MonadIO m => s -> Model s a b -> Managed (ViewT m b, Controller a) -> m s

ViewT could be nice to have too

[teh]

That would also be useful for running with Pipes.Safe's SafeT I think.

[Gabriella Gonzalez]

So this is explicitly designed to not require SafeT. The idea is that now you use Managed to deal with resource acquisition and release.