Dictionary optimizations:

Dictionary optimizations:

* Factored constant structure references out of the inner loops for
  PyDict_Next(), dict_keys(), dict_values(), and dict_items().
  Gave measurable speedups to each (the improvement varies depending
  on the sparseness of the dictionary being measured).

* Added a freelist scheme styled after that for tuples.  Saves around
  80% of the calls to malloc and free.  About 10% of the time, the
  previous dictionary was completely empty; in those cases, the
  dictionary initialization with memset() can be skipped.