The attached patch adds another version of cPickle.c's batch_dict(),
batch_dict_exact(), which is specialized for "type(x) is dict". This
provides a nice performance boost when pickling objects that use
dictionaries:

Pickle:
Min: 2.216 -> 1.858: 19.24% faster
Avg: 2.238 -> 1.889: 18.50% faster
Significant (t=106.874099, a=0.95)

Benchmark is at
http://code.google.com/p/unladen-swallow/source/browse/tests/performance/macro_pickle.py
(driver is  ../perf.py; perf.py was run with "--rigorous -b pickle").

This patch passes all the tests added in issue 5665. I would recommend
reviewing that patch first. I'll port to py3k once this is reviewed for
trunk.