Officially implement a clone macro

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At the bottom of http://gtk-rs.org/tuto/closures, they gave us this

macro_rules! clone {
    (@param _) => ( _ );
    (@param $x:ident) => ( $x );
    ($($n:ident),+ => move || $body:expr) => (
        {
            $( let $n = $n.clone(); )+
            move || $body
        }
    );
    ($($n:ident),+ => move |$($p:tt),+| $body:expr) => (
        {
            $( let $n = $n.clone(); )+
            move |$(clone!(@param $p),)+| $body
        }
    );
}

which would allow more seamless cloning.

let windows_clone = windows.clone();
button.connect_clicked(move |_| {

would become

button.connect_clicked(clone!(windows => move |_| {

Why is this good?

1. Less messy
2. No reason to think of an unique variable name every time

And I just thought having some official solution to this, whether it's as simple as copying the macro code, or if it's making some new syntax, it would be awesome to have :)

Start by reviewing the clone macro example in the gtk-rs closures tutorial linked by the issue. Determine whether an official solution should copy that macro or introduce new syntax; done requires a decided design and corresponding RFC implementation scope, but no repository file or test is named.