Two new built-in functions: issubclass() and isinstance(). Both take

Two new built-in functions: issubclass() and isinstance().  Both take
classes as their second arguments.  The former takes a class as the
first argument and returns true iff first is second, or is a subclass
of second.

The latter takes any object as the first argument and returns true iff
first is an instance of the second, or any subclass of second.

Also, change all occurances of pointer compares against
PyExc_IndexError with PyErr_ExceptionMatches() calls.