Fixed the len(d) == 1 size regression. Final performance of the patch
relative to trunk:

Using Unladen Swallow's perf.py -b pickle,pickle_dict on trunk:
pickle:
Min: 2.238 -> 1.895: 18.08% faster
Avg: 2.241 -> 1.898: 18.04% faster
Significant (t=282.066701, a=0.95)

pickle_dict:
Min: 2.163 -> 1.375: 57.36% faster
Avg: 2.168 -> 1.376: 57.50% faster
Significant (t=527.668441, a=0.95)


Performance for py3k:
pickle:
Min: 2.849 -> 2.790: 2.10% faster
Avg: 2.854 -> 2.796: 2.09% faster
Significant (t=27.624303, a=0.95)

pickle_dict:
Min: 2.121 -> 1.512: 40.27% faster
Avg: 2.128 -> 1.519: 40.13% faster
Significant (t=283.406572, a=0.95)


regrtest.py -uall test_xpickle passes all backwards-compatibility tests
for trunk, and all other tests run by regrtest.py on Linux pass.

Committed as r72909 (trunk), r72910 (py3k).