Add assertion checks for mkCoVar/mkTyVar

Add assertion checks for mkCoVar/mkTyVar

A type variable has a flag saying whether it is a *type* variable or
a *coercion* variable.  This patch adds assertions to check the flag.

And it adds fixes to places which were Wrong (and hence fired the
assertion)!

Also removed isCoVar from Coercion, since it's done by Var.isCoVar.