MSVC hardware-assisted popcount implementation

[jeking3]

In #25 support was added for hardware-assisted popcount.
In #33 it was noted that the implementation for MSVC failed to check for CPU support, and the MSVC documentation states the behavior is undefined in this case.
In #35 the MSVC implementation was disabled to resolve #33, which leaves the library performance in this respect the same as it was in 1.68.0.

This issue is a placeholder to come back around and clean up the implementation so it can leverage hardware assist safely with MSVC. There are some thoughts in the comments for #33 regarding alternate implementations that are useful when making a decision here.