extend stdlib to allow import of more packages (#1099)

* stdlib: extend stdlib to allow import of more packages

src/sync/mutex.go

@@ -74,3 +74,14 @@ type Locker interface {
 	Lock()
 	Unlock()
 }
+
+// RLocker returns a Locker interface that implements
+// the Lock and Unlock methods by calling rw.RLock and rw.RUnlock.
+func (rw *RWMutex) RLocker() Locker {
+	return (*rlocker)(rw)
+}
+
+type rlocker RWMutex
+
+func (r *rlocker) Lock()   { (*RWMutex)(r).RLock() }
+func (r *rlocker) Unlock() { (*RWMutex)(r).RUnlock() }