Remove or make even more optional the `url` crate in the next version.

[Thomas-airmatrix]

Outside of tests there is only a single use of url::Url in tungstenite:

impl<'a> IntoClientRequest for &'a Url {
    fn into_client_request(self) -> Result<Request> {
        self.as_str().into_client_request()
    }
}

impl IntoClientRequest for Url {
    fn into_client_request(self) -> Result<Request> {
        self.as_str().into_client_request()
    }
}

Which is just implementing some helper conversion traits.

Instead, we can just have users call as_str() on the Url directly if needed.

With http =1, http::uri::Uri has replaced most uses of the url crate. I have multiple project such that the only use of url is tungstenite.

[Its-Just-Nans]

This can be closed ?! (#419)