Note [Using coreView in mk_cast_ty]

Invariants (EQ3) and (EQ4) of Note [Respecting definitional equality] in
GHC.Core.TyCo.Rep must apply regardless of type synonyms. For instance,
consider this example (#19742):

   type EqSameNat = () |> co
   useNatEq :: EqSameNat |> sym co

(Those casts aren't visible in the user-source code, of course; see #19742 for
what the user might write.)

The type `EqSameNat |> sym co` looks as if it satisfies (EQ3), as it has no
nested casts, but if we expand EqSameNat, we see that it doesn't.
And then Bad Things happen.

The solution is easy: just use `coreView` when establishing (EQ3) and (EQ4) in
`mk_cast_ty`.