fix golint issues in core/hash (#487)

core/hash/hash.go

@@ -7,16 +7,19 @@ import (
 	"github.com/spaolacci/murmur3"
 )
 
+// Hash returns the hash value of data.
 func Hash(data []byte) uint64 {
 	return murmur3.Sum64(data)
 }
 
+// Md5 returns the md5 bytes of data.
 func Md5(data []byte) []byte {
 	digest := md5.New()
 	digest.Write(data)
 	return digest.Sum(nil)
 }
 
+// Md5Hex returns the md5 hex string of data.
 func Md5Hex(data []byte) string {
 	return fmt.Sprintf("%x", Md5(data))
 }