Tidy up coercions, and implement csel1, csel2, cselR

Tidy up coercions, and implement csel1, csel2, cselR

In preparation for implementing the PushC rule for coercion-swizzling
in the Simplifier, I had to inmplement the three new decomposition
operators for coercions, which I've called csel1, csel2, and cselR.

     co :: ((s1~t1) => r1) ~ ((s2~t2) => r2)
     ---------------------------------------
              csel1 co :: s1~s2

and similarly csel2, cselR.

On the way I fixed the coercionKind function for types of form
          (s1~t2) => r2
which currently are expressed as a forall type.

And I refactored quite a bit to help myself understand what is
going on.