Now that coercion variables mention types, a type-lambda binder can
have free variables.  This patch adjusts the free-variable finder
to take account of this, by treating Ids and TyVars more uniformly.

In addition, I fixed a bug in the specialiser that was missing a 
free type variable in a binder.  And a bug in tyVarsOfInst that
was missing the type variables in the kinds of the quantified tyvars.