使用 40 行代码实现一个最简化版本的 Koa
测试用例如下：
// Example
const app = new Application()
app.use(async (ctx, next) => {
console.log('Middleware 1 Start')
await next()
console.log('Middleware 1 End')
})
app.use(async (ctx, next) => {
console.log('Middleware 2 Start')
await next()
console.log('Middleware 2 End')
})
app.listen(7000)
// console.log
// Middleware 1 Start
// Middleware 2 Start
// Middleware 2 End
// Middleware 1 End