prime-factors stub should use u64 instead of u32

[petertseng]

As pointed out in #440 (comment) , the prime-factors tests use a number larger than the maximum u32. Thus, we should use u64 in the stub.

If we do not do this, students will receive the stub with u32 and be confused until they figure out to change it.

This is of course the immediate short-term fix. If you are interested in helping solve the long-term problem, please see #461.