Streaming HTML support

[zannabianca1997]

This is an initial implementation for supporting HTML streaming.

I found maud quite adapt in generating pieces of HTML, and the output of the html macro can easily be converted to an async stream.

By adding a single control flow @yield marking where to pass the buffer forward toward the client I was able to generate a streaming body. For now I implemented the support for Axum, as it is the only framework I am used to.

A brief example of the resulting code:

async fn slow_list() -> StreamingMarkup<impl Stream<Item = Markup> + Send + 'static> {
    streaming_html!(
        (DOCTYPE)
        html {
            head { title { "Slow list" } }
            body {
                ol {
                    @yield
                    @for i in 0..10 {
                        li {
                            ({sleep(Duration::from_secs(1)).await; format!("Slept for {}", i+1)})
                            @yield
                        }
                    }
                }
            }
        }
    )
}

This request is still a draft, as it is missing both tests and documentation. I think this feature would be more adapt to a sister/support crate, but I would have to copy all the parsing machinery.

Is this something that could be of interest?