Simplify implementation for -builtin, which does not need a fallback to
use strcmp as PyType_IsSubtype() 'just works' even when using multiple
modules (I think perhaps because SwigPyObject_stype->clientdata->pytype is
common across modules due to the implementation in SwigPyObject_Type()).
In the non-builtin case SwigPyObject is not a base type and usage is
different and when multiple modules are being used, SwigPyObject_Type()
returns two implementations of SwigPyObject which is solved in a hacky
way by comparing the types as strings when the pointer comparison fails.
Add import_callback test - tests %import and %callback to exercise
all of SwigPyPacked_Check(). Python only - the main callback example is
not widely tested and needs work in most of the other languages.
William S Fulton