Trac #13879 showed that when we were trying to solve

  (forall z1 (y1::z1). ty1)  ~  (forall z2 (y2:z2). ty2)

we'd end up spitting out z1~z2 with no binding site for them.
Those kind equalities need to be inside the implication.

I ended up re-factoring the code for solving forall-equalities.
It's quite nice now.