More ergonomic use_state hook

[lukechu10]

Problem
This is a continuation of #1026 (comment)

For the use_state hook, it would be more ergonomic if it only returned a struct rather than a tuple. It could be something like this:

#[derive(Clone)]
pub struct StateHandle<T> {
    value: Rc<T>,
}

impl<T> StateHandle<T> {
    /// Update the state and trigger a rerender.
    pub fn set(value: T) { }
}

impl<T> Deref for StateHandle<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target { }
}

It would be used like this:

let counter = use_state(|| 0);

html! {
    <button onclick=Callback::from(|_| counter.set(**counter + 1)) >{ "+1" }</button>
    <p>{ *counter }</p>
}

This way, when cloning the state into a closure (for callbacks), we only need to clone one Rc instead of two which should improve performance and ergonomics.

Even though this is a major change, it would be better to change it now rather then when Hooks are officially released.

Questionnaire

• I'm interested in fixing this myself but don't know where to start
• I would like to fix and I have a solution
• I don't have time to fix this right now, but maybe later

[pickfire]

    <button onclick=Callback::from(|_| counter.set(*counter + 1)) >{ "+1" }</button>
    <p>{ *counter }</p>

I believe this could be more ergonomic.

[ranile]

I implemented it in #1780. I think we should make use_reducer work in a similar way. Thoughts? I'll be trying to implement that soon in that PR.