func Sum64(data []byte) uint64 {
	d := &digest128{h1: 0, h2: 0}
	d.tail = d.bmix(data)
	d.clen = len(data)
	h1, _ := d.Sum128()
	return h1
}
Sum64 returns the MurmurHash3 sum of data. It is equivalent to the
following sequence (without the extra burden and the extra allocation):
hasher := New64()
hasher.Write(data)
return hasher.Sum64()
package murmur3
import (
	"hash"
)
var (
	_ hash.Hash   = new(digest64)
	_ hash.Hash64 = new(digest64)
	_ bmixer      = new(digest64)
)
type digest64 digest128
func New64(seed uint64) hash.Hash64 {
	d := (*digest64)(New128(seed, uint64(0)).(*digest128))
	return d
}
func (d *digest64) Sum(b []byte) []byte {
	h1 := d.h1
	return append(b,
		byte(h1>>56), byte(h1>>48), byte(h1>>40), byte(h1>>32),
		byte(h1>>24), byte(h1>>16), byte(h1>>8), byte(h1))
}
func (d *digest64) Sum64() uint64 {
	h1, _ := (*digest128)(d).Sum128()
	return h1
}