fetch support fetch_once() and fetch_later

[rebo]

Initial draft of fetch support.

Useful for fetching on component mount, or in response to an Event.

fetch_once fetches once straight away and deserializes to the given type (useful for fetching on mount). It automatically refreshes the view when the response is received.

fetch_later prepares a fetch and but only fetches on do_fetch() being called, useful if a fetch is needed in response to a button click.

Both have optional Msg on response.

Let me know if you have any comment on the API surface. The type parameters are a little busy but this should be able to be reduced in the future.

Simple example:

// fetches a user on mount
#[topo::nested]
fn fetch_on_mount_example() -> Node<Msg> {
    let fetch_user = fetch_once::<User, Model, _, _>("user.json", Msg::default());
    div![fetch_user.fetched_value()]
}

Fuller API login example as below:

// fetches a user on log-in click
#[topo::nested]
fn show_user_later() -> Node<Msg> {
    let fetch_user = fetch_later::<User, _>("user.json");

    if let Some(user) = fetch_user.fetched_value() {
        div![
            "Logged in User: ",
            user.name,
            div![a![
                attrs![At::Href=>"#logout"],
                "Log out",
                mouse_ev(Ev::Click, move |_| {
                    fetch_user.delete();
                    Msg::default()
                })
            ]]
        ]
    } else {
        div![a![
            attrs![At::Href=>"#login"],
            "Log in",
            mouse_ev(Ev::Click, move |_| {
                fetch_user.do_fetch::<Model, Msg, Node<Msg>>(Msg::default());
                Msg::default()
            })
        ]]
    }
}

and on mount:

#[topo::nested]
// fetches a user on mount
fn show_user() -> Node<Msg> {
    let fetch_user = fetch_once::<User, Model, _, _>("user.json", Msg::default());
    if let Some(user) = fetch_user.fetched_value() {
        div![
            "Logged in User: ",
            user.name,
            div![a![
                attrs![At::Href=>"#logout"],
                "Log out",
                mouse_ev(Ev::Click, move |_| {
                    fetch_user.delete();
                    Msg::default()
                })
            ]]
        ]
    } else {
        div![a![attrs![At::Href=>"#login"], "Log in"]]
    }
}

[MartinKavik]

I'm a simple user. I don't want to learn and remember fetch_once, fetch_later, .fetched_value, .delete(), do_fetch(, etc. Can I just reuse basic API and write something like:

#[topo::nested]
fn fetch_on_mount_example() -> Node<Msg> {
    let fetched_user = use_state(Option::<User>::default);
    do_once(async || {
        fetched_user.set(fetch("user.json").await.unwrap().json().await.ok()); // PR 353
    });
    div![fetched_user.map(|user| user.name)]
}

#[topo::nested]
fn show_user() -> Node<Msg> {
    let fetched_user = use_state(Option::<User>::default);
    div![
        div![
            "Load user",
            ev(Ev::Click, async || {
                fetched_user.set(fetch("user.json").await.unwrap().json().await.ok()); // PR 353
            } 
        ],
        fetched_user.map(|user| user.name),
    ]
}