Rename Bitwise::population_count to Bitwise::count_ones and add Bitwise::count_zeros

These are inspired by the [functions in the Julia standard library](http://docs.julialang.org/en/release-0.2/stdlib/base/#Base.count_ones).
auto merge of #12331 : bjz/rust/count-ones, r=alexcrichton

This is inspired by the [function naming in the Julia standard library](http://docs.julialang.org/en/release-0.2/stdlib/base/#Base.count_ones). It seems like a more self-explanatory name, and is more consistent with the accompanying methods, `leading_zeros` and `trailing_zeros`.