#458 Rename bit vector and related classes to shorter names

• I did the changes in multiple commits.
• I essentially replaced TwosComplement by Signed in the class names, and dropped Cif. I opted to keep BddBitVector rather than BddVector, as the concept of a 'bit vector' is more well-known, and this is a special kind of bit vector, namely a BDD one. I then changed various things in the code to make it all compatible again, rewrap code, etc.
• Note that this merge request is about renaming. It is not about re-discussing implementations of bit vectors, making optimizations to the code, making functional changes, domains supporting negative values, etc.

Addresses #458 (closed)