Look into fixing _SHL and _SHR

[mkilgore]

See some of the notes in #153. Basically, _SHL and _SHR only have one implementation in libqb.cpp, which takes a uint64_t. This causes some unexpected results due to type casting to uint64_t, but it's also not clear we can just fix it to work like _ROR and _ROL without it being a breaking change.

IMO a first good step would be to add some tests like the rotations tests for _SHL and _SHR that show the current behavior. It would explain what the behavior is now, and also act as a verification of what is different after any changes we make to fix it.