all: use a custom sync package

The sync package is strongly tied to the runtime, so it's easier to
implement a new one. Besides, it's pretty big so it's better to replace
it.

src/sync/once.go

@@ -0,0 +1,16 @@
+package sync
+
+type Once struct {
+	done bool
+	m    Mutex
+}
+
+func (o *Once) Do(f func()) {
+	o.m.Lock()
+	defer o.m.Unlock()
+	if o.done {
+		return
+	}
+	o.done = true
+	f()
+}