Add 64bit popcnt, make use of POPCNT instruction

By using Intel's POPCNT instruction, we can speed up popcnt by a fair amount. I have not done measurements myself, but according to benchmarks others have made, it should run at least 3 times as fast as non-sse implementations.

Furthermore, this PR will also allow to pass 64bit integers to popcnt.
Needs a fallback for non-x86 (e.g. ARM).