Provide a simple `async fn send` method for better interoperability with `futures-lite` based facilities

[stackinspector]

Smol(futures-lite)'s guys think that futures_sink::Sink (till v0.3) is a legacy API and excluded related APIs in futures-lite intentionally. They prefer a simple async fn send API.
Before a new simple Sink API is normalized, I suggest considering implementing a simple async fn send method:

impl<S> WebSocketStream<S> {
    /// Simple send method to replace `futures_sink::Sink` (till v0.3).
    pub async fn send(&mut self, msg: Message) -> Result<(), WsError>
    where
        S: AsyncRead + AsyncWrite + Unpin,
    {
        // Example to make it work. A simpler internal way can be used to implement it.
        SinkExt::send(self, msg).await
    }
}

In an ideal world, the send method would be called even if the SinkExt was introduced. I'm not sure if there would be additional problems if a method name is duplicated with a method in trait.
cc smol-rs/futures-lite#111 smol-rs/futures-lite#112

[stackinspector]

smol/futures-lite#112 has been merged.
Final document: https://github.com/smol-rs/futures-lite/blob/master/FEATURES.md

[sdroege]

I'm also not a fan of the Sink API. Having a simple send() function like this would seem like a good idea. Do you want to provide a PR for adding it?

[stackinspector]

I tried but I realized I'm not very good at handling details of async primitives. Considering code robustness, it might be a good idea to assign community members who's more experienced.