Speed up `SparseBitMatrix` use in `RegionValues`.
In practice, these matrices range from 10% to 90%+ full once they are
filled in, so the dense representation is better.
This reduces the runtime of Check Nll builds of `inflate` by 32%, and
several other benchmarks by 1--5%.
It also increases max-rss of `clap-rs` by 30% and a couple of others by
up to 5%, while decreasing max-rss of `coercions` by 14%. I think the
speed-ups justify the max-rss increases.
r? @nikomatsakis
a57d5d7b25