Remove support for void* return types

Assuming that a void pointer is a reference SV is fragile. The intention
of the mapping was to support bindings for the To_Host method. But
calling To_Host from the host language would simply return an identical
object, so I can't see a need to bind this method.
Function parameter and return type check

Move parameter and return type check from a static function in
CFCPerlClass to CFCPerlSub. Allow types which are either objects or
primitives.
Separate test whether to create method bindings

Move the code that checks whether bindings for a method should be
generated to a separate function.
Consolidate invalid callback check

Use the same function to check for invalid callbacks as for method
XSUBs.

Ultimately, we need a mechanism to disable the autogeneration of XSUBs
and callbacks separately. For now, it seems safer to keep callbacks for
excluded methods.