const runtime.numSpanClasses

4 uses

	runtime (current package)
		mcache.go#L44: 	alloc [numSpanClasses]*mspan // spans to allocate from, indexed by spanClass
		mgcsweep.go#L59: 	numSweepClasses            = numSpanClasses * 2
		mheap.go#L206: 	central [numSpanClasses]struct {
		mheap.go#L535: 	numSpanClasses = _NumSizeClasses << 1