- make the locked decorator easier to inline by not creating a new closure every time.
- use builtin dicts instead of the pure-python BTrees (only under pypy)

"Transaction",                  AFTER    BEFORE
"Add 3000 Objects",             10014      8777
"Update 3000 Objects",          12199      8805
"Read 3000 Warm Objects",        4294      3699
"Read 3000 Cold Objects",        4264      3676
"Read 3000 Hot Objects",        79764     58732
"Read 3000 Steamin' Objects", 2667283   2574508