fixed getModelValue to only query the value of leaves and evaluate more complex bv terms
Added check for infinite lemma loop
Fixes for getModelVal in bv theory
Improved speed of no redundant lemma assertion by using hash set
flatten or nodes
generalize to handle and
boolean flatten rewrite: dont re-rewrite
boolean flatten: bug fix in dfs search

(this is not intended to (and doesn't) address the issue with
NodeBuilder limit)
bool flatten: node num_children workaround