Memory optimization

[lucaneg]

Description
This addresses #248:

• provide a basic block-based fixpoint algorithm to improve efficiency
  • use watchpoint predicates to store approximation of statements when using basic blocks, and unwind the results when asked for approximation of non-watchpoint statements
  • dumping will not unwind the results
• apply widening only on widening points
• remove satisfies invocation on edges, and rely only on assume
• add eval(exp) to environments instead of tracking what is left on the stack
• avoid duplicates in working sets used in fixpoints to improve efficiency

Closes #248