Change Render and RenderOnce to return Markup instead

[lambda-fairy]

In theory the existing signatures of Render/RenderOnce allow for more efficient code. But in practice we often end up just invoking html! and pushing that to the buffer. If this is the more common use case, then we should have these trait methods return Markup directly instead.

Something like this:

trait Render {
    fn render(&self) -> Markup;

    // what .render() used to be
    fn render_to(&self, buffer: &mut String) {
        buffer.push_str(&self.render());
    }
}

// similarly for RenderOnce

Note that we still need .render_to() to optimize the blanket impl for T: Display, because write!(s, "{}", x) is much faster than x.to_string().

Example:

// shorthand for linking to a stylesheet
struct Stylesheet<S>(pub S);
impl<S: AsRef<str>> Render for Stylesheet<S> {
    fn render(&self) -> Markup {
        html! {
            link rel="stylesheet" type="text/css" href=(self.0.as_ref()) /
        }
    }
}