Consolidate invalid callback check
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.

rectang/lucy-clownfish,apache/lucy-clownfish,rectang/lucy-clownfish,apache/lucy-clownfish,nwellnhof/lucy-clownfish,nwellnhof/lucy-clownfish,rectang/lucy-clownfish,rectang/lucy-clownfish,nwellnhof/lucy-clownfish,apache/lucy-clownfish,nwellnhof/lucy-clownfish,apache/lucy-clownfish,nwellnhof/lucy-clownfish,rectang/lucy-clownfish,nwellnhof/lucy-clownfish,nwellnhof/lucy-clownfish,apache/lucy-clownfish,rectang/lucy-clownfish,apache/lucy-clownfish,apache/lucy-clownfish,rectang/lucy-clownfish,nwellnhof/lucy-clownfish,rectang/lucy-clownfish,apache/lucy-clownfish,rectang/lucy-clownfish