add vendoring

vendor/github.com/jhillyerd/enmime/internal/stringutil/uuid.go

@@ -0,0 +1,24 @@
+package stringutil
+
+import (
+	"fmt"
+	"math/rand"
+	"sync"
+	"time"
+)
+
+var uuidRand = rand.New(rand.NewSource(time.Now().UnixNano()))
+var uuidMutex = &sync.Mutex{}
+
+// UUID generates a random UUID according to RFC 4122.
+func UUID() string {
+	uuid := make([]byte, 16)
+	uuidMutex.Lock()
+	_, _ = uuidRand.Read(uuid)
+	uuidMutex.Unlock()
+	// variant bits; see section 4.1.1
+	uuid[8] = uuid[8]&^0xc0 | 0x80
+	// version 4 (pseudo-random); see section 4.1.3
+	uuid[6] = uuid[6]&^0xf0 | 0x40
+	return fmt.Sprintf("%x-%x-%x-%x-%x", uuid[0:4], uuid[4:6], uuid[6:8], uuid[8:10], uuid[10:])
+}