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.
What I've being doing in my integer library is leaving off the sizeof constraint and allowing smaller sizes to take the 32 bit code path (via implicit cast.) i.e. ```static if (T.sizeof <= 4) return popcnt_32(x);```