Better coroutine

[schell]

This provides Coroutine with a return value, which makes it more composable (and actually useful). It also provides runC' which does not strip the effect from the typelist, allowing more complicated branching schemes (like step-wise dueling coroutines).

[trskop]

This is related to issue #10.

[trskop]

Thanks for making coroutines usable. We kind of reached an impasse with it.

I've renamed runC' to interposeC. There's an issue with naming things by adding prime without having a fixed meaning for it. Hopefully this wont break any code you may possibly have.

[schell]

Thank you! I'm in favor of interposeC :) 👍