- Got rid of instance_cmp(); refactored instance_compare().

- Added instance_richcompare() which calls __lt__() etc.

Some unrelated stuff mixed in:

- Aligned comments in various large struct initializers.

- Better test to avoid recursion if __coerce__ returns self as the
  first argument (this is an unrelated fix by Neil Schemenauer!).

- Style nit: don't use Py_DECREF(Py_NotImplemented); use
  Py_DECREF(result) -- it just looks better. :-)