These affected only quality 11, and now it does not make sense
to disable block splitting or context modeling because most of
the time is spent in zopfli anyway.
Now all speed vs size compromises are controlled by the quality param.
* Cluster at most 64 histograms at a time in the first
round of clustering.
* Use a faster histogram cost estimation function.
* Don't compute the log2(total) multiple times in the
block splitter.
Disable all slow features for quality <= 9 (literal cost modeling,
dictionary, context modeling, advanced block splitting).
Change vector<Command> arguments of internal functions
to Command* and size_t.
