Prioritise nominal equalities

The main payload of this patch is

* Prioritise nominal equalities in the constraint solver. This
  ameliorates the incompleteness of solving for representational
  constraints over newtypes: see #24887.

   See (EX2) in Note [Decomposing newtype equalities] in
   GHC.Tc.Solver.Equality

In doing this patch I tripped over some other things that I refactored:

* Move `isCoVarType` from `GHC.Core.Type` to `GHC.Core.Predicate`
  where it seems more at home.

* Clarify the "rewrite role" of a constraint.  I was very puzzled
  about what the role of, say `(Eq a)` might be, but see the new
  Note [The rewrite-role of a constraint].

  In doing so I made predTypeEqRel crash when given a non-equality.
  Usually it expects an equality; but it was being mis-used for
  the above rewrite-role stuff.