#458 Add BddBitVector.shrink and use it in CifToBddConverter to prevent unnecessary length increases

• This is the last step in #458 (closed).
• Best to review per commit.
• End-user visible changes:
  • Improved performance for CIF to BDD conversion in certain cases, where - (unary), - (binary), + (binary), div, or mod operations are involved.
• Other changes:
  • Added BddBitVector.shrink.

Closes #458 (closed)