func round64(h, v uint64) uint64 {
	h += v * prime64x2
	h = rotl64_31(h)
	h *= prime64x1
	return h
}
func round64(h, v uint64) uint64 {
	h += v * prime64x2
	h = rotl64_31(h)
	h *= prime64x1
	return h
}
func round64 ( h , v uint64 ) uint64 { h += v * prime64x2 h = rotl64_31 ( h ) h *= prime64x1 return h }
// borrowed from cespare
borrowed from cespare