Consolidate bitCount and popcount

This removes the bitCount function from util.go that appeared to be duplicate of popcount code in popcount.go.
The test suite still passes.
I have not done anything about checking to see if popcount.go is correctly hooking into the optimized implementations.
I noticed there was other code that was calling into popcount.go functions and casting to int, so I followed the same pattern.
Please feel free to ask for any revisions that would make this easier to merge or more consistent with your vision for the project.