WIP: Allow for returning counterexamples

[arianvp]

Fixes #209

This is still a WIP. but the first step is completed (adding a type parameter to Property)

[arianvp]

Need some advice here.

Considering TestT is the actual thing that is returning a Failure, by means of an ExceptT I actually think the type
of PropertyT needs to be more like:

newtype TestT e m a
newtype PropertyT e m a

data Property e = { PropertyT e m () }

reflecting that a PropertyT can return a counterexample e.

but that actually seems to be a bit of a bigger refactoring, as I'll need to refactor MonadTest to carry
around e too

but then functions like eval, evalEither and evalM become unimplementable... so I'm kind of stuck and it becomes a bit of a larger refactoring than I would've hoped