Add shift operations on BitSet

[linasm]

Add bitwise shift (<< and >>) operations to BitSet collection.
Motivation:

• completeness of bitwise operation support
• applicable in Dynamic Programming solutions to the Subset Sum / Knapsack types of problems
• feature parity with C++ std::bitset: https://en.cppreference.com/w/cpp/utility/bitset/operator_ltltgtgt

Note: << can be emulated with .map(_ + shiftBy) and >> with .collect { case b if b >= shiftBy => b - shiftBy }, but performance difference is two orders of magnitude.

I have some existing code for it, will be able to submit a PR soon.

[linasm]

Solved in #43.