New version from Jim Fulton to fix a problem that Eric Raymond ran

New version from Jim Fulton to fix a problem that Eric Raymond ran
into.  Jim writes:

The core dump was due to a C decrement operation
in a 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. :)

I tested these changes in *my* cPickle, which
doesn't have the new Unicode stuff.