Reactive Slick

Here's the ongoing work on Reactive Slick.

/cc @cvogt for review later
Master/slave handling can be implemented by the user on top of Slick. We could add out-of-the-box support for it later. Actually, if this is the only thing for which we would use the effect types, we might as well drop them entirely and use flags instead. There's no compelling reason to encode this distinction in the types.

I'll release M1 this week to get some feedback over the holidays. I think the basic model is sound and the new API can be non-experimental in the next release, but there's still some time to make a final decision.