Rather than switching to CPython's dict implementation, which would
be quite a bit of code changes (since we use our internal dict interface
a lot), change our current dict implementation to more-closely match CPython's.

In particular, change the growth strategy and probing strategy to match cpython's.

I'm not sure how I feel about this -- there are still a number of differences.
In theory this is still an improvement over the current status quo (fewer ordering
differences), but I don't know if we need to go all the way and make sure to have
zero ordering differences.