Using faster popcnt function in pure Go

The following pure Go popcnt function is nearly as fast as the AMD64 assembly version. It is significantly faster than the version found on Wikipedia that we were using previously.

On a recent Haswell processor (2013), there is no benefit at all to using the assembly version, as I can demonstrate:

	$ go test -bench=BenchmarkCount
	PASS
	BenchmarkCount	 1000000	      2424 ns/op
	ok  	_/home/dlemire/CVS/github/bitset	2.455s
	$ vim popcnt_asm.go
	$ git diff
	diff --git a/popcnt_asm.go b/popcnt_asm.go
	index ef75be7..9825bb6 100644
	--- a/popcnt_asm.go
	+++ b/popcnt_asm.go
	@@ -6,7 +6,7 @@ package bitset
	
	 func hasAsm() bool
	
	-var useAsm = hasAsm()
	+var useAsm = false //hasAsm()
	
	 //go:noescape
	
	$ go test -bench=BenchmarkCount
	PASS
	BenchmarkCount	 1000000	      2464 ns/op
	ok  	_/home/dlemire/CVS/github/bitset	2.495s