z: use MemHashString and xxhash.Sum64String (#153)

Use `MemHashString` and `xxhash.Sum64String` for KeyToHash
which is faster than `MemHash` and `Sum64`. See
#153 for benchmark results.

z/rtutil_test.go

@@ -13,9 +13,24 @@ import (
 func BenchmarkMemHash(b *testing.B) {
 	buf := make([]byte, 64)
 	rand.Read(buf)
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = MemHash(buf)
+	}
+	b.SetBytes(int64(len(buf)))
+}
+
+func BenchmarkMemHashString(b *testing.B) {
+	s := "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua."
+
+	b.ReportAllocs()
+	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
-		MemHash(buf)
+		_ = MemHashString(s)
 	}
+	b.SetBytes(int64(len(s)))
 }
 
 func BenchmarkSip(b *testing.B) {

z/z.go

@@ -36,8 +36,7 @@ func KeyToHash(key interface{}) (uint64, uint64) {
 	case uint64:
 		return k, 0
 	case string:
-		raw := []byte(k)
-		return MemHash(raw), xxhash.Sum64(raw)
+		return MemHashString(k), xxhash.Sum64String(k)
 	case []byte:
 		return MemHash(k), xxhash.Sum64(k)
 	case byte: