Provide a stateless version of MockT

[pepeiborra]

As described here the class of monads for which StM m a ~ a holds enjoy better compositionality properties than the ones that do not.

This property does not currently hold for MockT, as StM (MockT f) a ~ (a, [WithResult f]). This is a consequence of building MockT f on top of StateT [WithResult f] m, and it limits the applicability of MockT in monad stacks which require a stateless MonadTransControl instance. This is not an academic concern - it happens to be quite common these days.

To address this,MockT can be defined on top of ReaderT (IORef [WithResult f]) m, which possesses a MonadIO instance as long given MonadIO m. Or more generally using MutVar from the primitive package.

Since this incurs in additional constraints for the base monad, it's not a backwards compatible change. An alternative is to add a Control.Monad.Mock.Stateless module with a one-for-one replacement for MockT.

[pepeiborra]

I'm happy to work on this if the contribution would be considered