Destructuring let (0.7)

[Jinxit]

This PR adds a new let() syntax to components, while also keeping let: alive for backwards compatibility.

Example usage:

view! {
    <For
        each=move || data.get()
        key=|n| *n
        // stores the item in each row in a variable named `data`
        let(data)
    >
        <p>{data}</p>
    </For>
}

This also allows for destructuring of tuples and structs, as it's just directly embedded in the children function's arguments.

view! {
    <For
        each=move || data.get()
        key=|n| *n
        // stores the item in each row in a variable named `data`
        let((tup1, tup2)) // note the extra parentheses for destructuring
    >
        <p>{tup1} {tup2}</p>
    </For>
}

I haven't gone through all the examples and changed the usage as I wasn't sure what you wanted to present as the default. Would you like me to do that?

[benwis]

Lovely, thanks for the PR