Static vdom branches diffing optimization

[maciejhirsz]

Consider scenario such as:

html! {
    <h1 class="header"><strong>{ "Hello world" }</strong></h1>
    { for some_list }
}

The <h1> tag is never changing, but it still has to be diffed on every render. It should be relatively easy for the html! macro to identify that the entire branch of the DOM is static, and produce some unique id for it at compile time (u64 fnv hash of the whole TokenStream would be the most naive), and then attach it at the root VTag of the <h1> (VTag is already boxed, so another field there shouldn't make much of a difference).

On runtime the PartialEq could then check if the Option<u64> of the hashes match, and if so short circuit the entire diff.