Major refactoring of django.dispatch with an eye towards speed. The net result is that signals are up to 90% faster.
Though some attempts and backwards-compatibility were made, speed trumped compatibility. Thus, as usual, check BackwardsIncompatibleChanges for the complete list of backwards-incompatible changes.
Thanks to Jeremy Dunck and Keith Busell for the bulk of the work; some ideas from Brian Herring's previous work (refs #4561) were incorporated.
Documentation is, sigh, still forthcoming.
Fixes #6814 and #3951 (with the new dispatch_uid argument to connect).
