Implement BVSHL, BVLSHR and BVASHR bit-shift by expression operators from CVC3.
Add test cases for BVSHL, BVLSHR and BVASHR.
Merge pull request #25 from khooyp/implement-cvc-varshifts

Implement BVSHL, BVLSHR and BVASHR bit-shift by expression operators from CVC3.