Note [Coercion evidence only]

Class constraints etc give rise to /term/ bindings for evidence, and
we have nowhere to put term bindings in /types/.  So in some places we
use CoEvBindsVar (see newCoTcEvBinds) to signal that no term-level
evidence bindings are allowed.  Notably ():

  - Places in types where we are solving kind constraints (all of which
    are equalities); see solveEqualities

  - When unifying forall-types