This adds an expert option to allow regular expressions to appear
anywhere in constraints, including in datatype fields, or as arguments
to uninterpreted functions.
This is required for reasoning about the correctness of CPC for proof
rules involving regular expressions.
This moves term statistics collection from Solver to TermManager.
TermManager and Solver now both provide `getStatistics()`, the former
captures API level term statistics, the latter internal solver
statistics.
This also adds iterator capabilities to the Python Statistics class.
