In an AbsBinds, the 'dicts' can include EqInsts

In an AbsBinds, the 'dicts' can include EqInsts

An AbsBinds abstrats over evidence, and the evidence can be both
Dicts (class constraints, implicit parameters) and EqInsts (equality
constraints).  So we need to
  - use varType rather than idType
  - use instToVar rather than instToId
  - use zonkDictBndr rather than zonkIdBndr in zonking

It actually all worked before, but gave warnings.