Make the treatment of equalities more uniform

Make the treatment of equalities more uniform

This patch (which is part of the fix for Trac #2018) makes coercion variables
be handled more uniformly.  Generally, they are treated like dictionaries
in the type checker, not like type variables, but in a couple of places we
were treating them like type variables.  Also when zonking we should use
zonkDictBndr not zonkIdBndr.