统一生成器接口

由于内部隐式rewind，需要先调用Generator::current获取当前value，而直接调用Generator::send会跳到第二次yield。

1. send方法参考
2. 生成器参考

<?php
class Gen
{
    public $isfirst = true;
    public $generator;
    public function __construct(\Generator $generator)
    {
        $this->generator = $generator;
    }
    public function valid()
    {
        return $this->generator->valid();
    }
    public function send($value = null)
    {
        if ($this->isfirst) {
            $this->isfirst = false;
            return $this->generator->current();
        } else {
            return $this->generator->send($value);
        }
    }
}