A bunch of functions are now properly implemented in abstract.c, and

A bunch of functions are now properly implemented in abstract.c, and
the code here becomes much simpler.  In particular: abs(), divmod(),
pow(), int(), long(), float(), len(), tuple(), list().

Also make sure that no use of a function pointer gotten from a
tp_as_sequence or tp_as_mapping structure is made without checking it
for NULL first.

A few other cosmetic things, such as properly reindenting slice().