Implement worker execution and task handling

- In pool.go, added a call to the worker's run method after submitting a task to it in the Submit method.
- In worker.go, implemented the run method for the Worker struct. This method starts a goroutine that continuously receives tasks from the worker's task channel, executes them, pushes the worker back to the pool, and decrements the pool's running count.

Signed-off-by: Daniel Hu <tao.hu@merico.dev>

pool.go

@@ -28,6 +28,7 @@ func (p *Pool) Submit(task Task) {
 			}
 		}
 		worker.task <- task
+		worker.run()
 		p.running++
 	} else {
 		// TODO: handle the case when the pool is full

worker.go

@@ -15,6 +15,21 @@ type Worker struct {
 	lastUsedTime time.Time
 }
 
+func (w *Worker) run() {
+	go func() {
+		for task := range w.task {
+			if task == nil {
+				return
+			}
+			task()
+			w.pool.lock.Lock()
+			w.pool.workers.Push(w)
+			w.pool.running--
+			w.pool.lock.Unlock()
+		}
+	}()
+}
+
 type WorkerStack interface {
 	Push(w *Worker)
 	Pop() *Worker