[IMP] component: refactor rendering pipeline

This commit introduces a brand new rendering system based on a fiber
class and a scheduler.

closes #330

src/component/scheduler.ts

@@ -0,0 +1,50 @@
+import { Fiber } from "./fiber";
+
+// scheduler
+interface Task {
+  fiber: Fiber;
+  callback: () => void;
+}
+
+export const scheduler = {
+  tasks: [] as Task[],
+  isRunning: false,
+
+  addFiber(fiber, callback) {
+    this.tasks.push({ fiber, callback });
+    if (this.isRunning) {
+      return;
+    }
+    this.scheduleTasks();
+  },
+  flush() {
+    let tasks = this.tasks;
+    this.tasks = [];
+    tasks = tasks.filter(task => {
+      if (task.fiber.isCancelled) {
+        return false;
+      }
+      if (task.fiber.counter === 0) {
+        task.callback();
+        return false;
+      }
+      return true;
+    });
+    this.tasks = tasks.concat(this.tasks);
+  },
+  processTasks() {
+    this.flush();
+    if (this.tasks.length > 0) {
+      this.scheduleTasks();
+    } else {
+      this.isRunning = false;
+    }
+  },
+
+  scheduleTasks() {
+    this.isRunning = true;
+    this.requestAnimationFrame(() => this.processTasks());
+  },
+
+  requestAnimationFrame: requestAnimationFrame.bind(window)
+};