Use explicit casting in cuckoocache's `compute_hashes(...)` to clarify integer conversion.

I discussed this code with the code's author @JeremyRubin who suggested patching it to avoid any confusion.

At least one static analyzer incorrectly warns about a shift past bitwidth (UB) here, so this patch will help avoid confusion for human reviewers and static analyzers alike :-)

make check
test_runner.py