Fix to empty response to (get-assignment).

Thanks to David Cok for reporting this issue.
Fixed "success" response to (push N) / (pop N) with N > 1.

Thanks to David Cok for reporting this issue.
Disallow construction of (_ BitVec 0).

Thanks to David Cok for reporting this issue.
A couple of fixes to the get-option command for compliance with SMT-LIB.

Thanks to David Cok for reporting this issue.
Better error on illegal (pop N); also more compliant SMT-LIB error messages in some places

Thanks to David Cok for reporting these issues.