Remove `STRef`, unify with `Ref`

[garyb]

Now there's the Global region, MonadST, etc. we can do away with having two Ref types:

• Rework Ref in purescript-refs to include Region (basically, STRef -> Ref)
• Introduce a Ref.Lifted module where the operations have a MonadST constraint so they can be used in Effect without friction.
• Remove STRef from purescript-st
• Remove the MonadRec instance currently in purescript-st, as it currently is implemented with STRef. Implement it in purescript-tailrec instead (implementation is basically identical to the Effect instance already in there).

I've done all this locally to check it works out 🙂.

I'd like to make this change when we do the updates for PS 0.14 unless there is a strong feeling we shouldn't for some reason. Aside from some module stuff moving around the main breaking difference is that existing Refs will need to become Ref Global.