Implement behavior for minimize() and full_sweep() as discussed on
zodb-dev.  minimize() ghostifies all unmodified objects.  full_sweep()
with age==0 is the same as minimize(), otherwise it's the same as
incrgc().

Reformat and/or reindent lots of code.

Use PyObject_Compare() instead of PyObject_Cmp() because it has a
simpler return value.

Fix a few more PyDict_SetItem() and PyDict_DelItem() calls to make
correct check for error return.