* 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.
