do not use fmt.Sprintf() in UUID.String()

A single array is used as a buffer and "encoding/hex".Encode() hex
encodes directly its final destination.  Loops are avoided because they
are simple enough to unroll manually.

uuid.go

@@ -7,6 +7,7 @@ package uuid
 import (
 	"bytes"
 	"crypto/rand"
+	"encoding/hex"
 	"fmt"
 	"io"
 	"strings"
@@ -81,9 +82,17 @@ func (uuid UUID) String() string {
 	if uuid == nil || len(uuid) != 16 {
 		return ""
 	}
-	b := []byte(uuid)
-	return fmt.Sprintf("%08x-%04x-%04x-%04x-%012x",
-		b[:4], b[4:6], b[6:8], b[8:10], b[10:])
+	var buf [36]byte
+	hex.Encode(buf[:], uuid[:4])
+	buf[8] = '-'
+	hex.Encode(buf[9:13], uuid[4:6])
+	buf[13] = '-'
+	hex.Encode(buf[14:18], uuid[6:8])
+	buf[18] = '-'
+	hex.Encode(buf[19:23], uuid[8:10])
+	buf[23] = '-'
+	hex.Encode(buf[24:], uuid[10:])
+	return string(buf[:])
 }
 
 // URN returns the RFC 2141 URN form of uuid,