Separate testing and input generation concepts

[jacobstanley]

I got stuck implementing parallel state machine testing as Test was not MonadBaseControl so I couldn't fork it to a new thread. This change makes a separation between things that support testing (Test) and things that support testing and can generate input (TestGen). This means that we can fork a thread in Test even though we can't do so for TestGen. It also clears up the strange ResourceT behaviour somewhat.

I'm not really happy with the names Test and TestGen, would probably prefer Test and Property, but then I don't know what to call the top level testable thing.

I'm also tossing up on the names of the eval functions, I named them this way so that evalExceptT looks like evalStateT from transformers. But really eval itself is more similar to evaluate in base:Control.Exception

! @charleso, I wouldn't mind a discussion about this some time

[charleso]

👍