Optimize LivenessAnalysis

We were caching certain liveness results (only crawled each BB once
looking for name use/defs), but we were still traversing the entire
CFG for every liveness query.
Now, each query does a single full CFG scan, and computes the results
for all BBs and stores them.

Reduces the time to 'import re' from about 13s to under 2s.
Merge branch 'master' of https://github.com/dropbox/pyston