Implement generator expressions by converting to generator functions
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.
ported objmodel.cpp to use rewriter2 exclusively, deprecates rewriter1
removed rewriter1
fixed a bunch of bugs