sync: move Mutex to internal/task

The mutex implementation needs a different implementation once support
for threading lands. This implementation just moves code to the
internal/task package to centralize these algorithms.

src/internal/task/mutex-cooperative.go

@@ -0,0 +1,43 @@
+package task
+
+type Mutex struct {
+	locked  bool
+	blocked Stack
+}
+
+func (m *Mutex) Lock() {
+	if m.locked {
+		// Push self onto stack of blocked tasks, and wait to be resumed.
+		m.blocked.Push(Current())
+		Pause()
+		return
+	}
+
+	m.locked = true
+}
+
+func (m *Mutex) Unlock() {
+	if !m.locked {
+		panic("sync: unlock of unlocked Mutex")
+	}
+
+	// Wake up a blocked task, if applicable.
+	if t := m.blocked.Pop(); t != nil {
+		scheduleTask(t)
+	} else {
+		m.locked = false
+	}
+}
+
+// TryLock tries to lock m and reports whether it succeeded.
+//
+// Note that while correct uses of TryLock do exist, they are rare,
+// and use of TryLock is often a sign of a deeper problem
+// in a particular use of mutexes.
+func (m *Mutex) TryLock() bool {
+	if m.locked {
+		return false
+	}
+	m.Lock()
+	return true
+}