Generate a pointer superclasses if specified by the user

(Just so you can give me advice)

When trying to generate the bindings for libetpan, I need to specify a superclass for a `clist *` structure. At the moment melange generates the superclasses for the referenced struct only, this doesn't allow a nice integration between dylan and the library. For example when implemeting the f-i-p.

In this patch I generate a temporary pointer type, then a new c type that has as parents the specified superclasses and the temporary pointer type.
This should do the trick, at least from a quick test I did by manually changing the generated code.