use async scope instead of unchecked borrow

[ant32]

simplify the code and library usage by copying the idea from https://crates.io/crates/async-scoped

this is a breaking change in that this now needs to be replaced with self as in the following example.

#[php_class]
struct Client {
    client: reqwest::Client,
}

#[php_async_impl]
impl Client {
    pub fn init() -> PhpResult<u64> {
        EventLoop::init()
    }
    pub fn wakeup() -> PhpResult<()> {
        EventLoop::wakeup()
    }
    pub fn new() -> Self {
        let client = reqwest::ClientBuilder::new().build().unwrap();
        Self {client}
    }
    pub async fn get(&mut self, url: &str) -> anyhow::Result<String> {
        Ok(self.client.get(url).send().await?.text().await?)
    }
}