macro invocation in load_pop.  (BAD)

I fixed this by moving the decrement outside
the macro call.

I added a comment to load_pop and load_mark to document the fact that
cPickle separates the unpickling stack into two separate stacks, one
for objects and one for marks.

I also moved some increments out of some macro calls (PyTuple_SET_ITEM
and PyList_SET_ITEM).  This wasn't necessary, but made me feel better.