Note [No top-level newtypes on RHS of representational equalities]

Suppose we're in this situation:

 work item:  [W] c1 : a ~R b
     inert:  [G] c2 : b ~R Id a

where
  newtype Id a = Id a

We want to make sure canEqCanLHS sees [W] a ~R a, after b is rewritten
and the Id newtype is unwrapped. This is assured by requiring only rewritten
types in canEqCanLHS *and* having the newtype-unwrapping check above
the tyvar check in can_eq_nc.

Note that this only applies to saturated applications of newtype TyCons, as
we can't rewrite an unsaturated application. See for example T22310, where
we ended up with:

  newtype Compose f g a = ...

  [W] t[tau] ~# Compose Foo Bar