Note [Newtype arity]

Non-recursive newtypes are transparent, and should not get in the way.
We do (currently) eta-expand recursive newtypes too.  So if we have, say

        newtype T = MkT ([T] -> Int)

Suppose we have
        e = coerce T f
where f has arity 1.  Then: etaExpandArity e = 1;
that is, etaExpandArity looks through the coerce.

When we eta-expand e to arity 1: eta_expand 1 e T
we want to get:                  coerce T (\x::[T] -> (coerce ([T]->Int) e) x)

  HOWEVER, note that if you use coerce bogusly you can ge
        coerce Int negate
  And since negate has arity 2, you might try to eta expand.  But you can't
  decompose Int to a function type.   Hence the final case in eta_expand.