Implement Shl and Shr (<< and >> operators)

[ghcs27]

Bitvec already provides shift_left and shift_right methods for in-place bitshifts, but it would be nice to provide operators which return the shifted BitVec/BitArray. This would also make it easier to translate code written with raw integers to cleaner bitvec code.

[ThibaultLemaire]

But make sure they're consistent with Rust's implementation on primitive types. (i.e. Shl moves bits away from the 0 index/least significant bit)