Fix optimisation of InstCo

It turned out (#25387) that the fix to #15725 was not quite right:

  commit 48efbc04
  Date:   Mon Oct 15 10:25:02 2018 +0200

    Fix #15725 with an extra Sym

Optimising InstCo is quite subtle, and the invariants surrounding
the LiftingContext in the coercion optimiser were not stated explicitly.

This patch refactors the InstCo optimisation, and documents these
invariants.  See
  * Note [Optimising InstCo]
  * Note [The LiftingContext in optCoercion]

I also did some refactoring of course:

* Instead of a Bool swap-flag, I am not using GHC.Types.Basic.SwapFlag

* I added some invariant-checking the coercion-construction functions
  in GHC.Core.Coercion.Opt.  (Sadly these invariants don't hold during
  typechecking, becuase the types are un-zonked, so I can't put these
  checks in GHC.Core.Coercion.)