Use nightly feature arbitrary_self_types

[PoignardAzur]

This is a showcase-only PR and isn't meant to be merged. This PR tries to show how the soon-to-be stabilized arbitrary_self_types will impact Masonry's codebase.

Note that it's a quick-and-dirty port; there's a ton of places in the code that end up calling Foobar::method(&mut foobar, ...) that could easily be refactored to foobar.method(...), I just didn't take the time to refactor them all.

[PoignardAzur]

Here's an example of what the user code ends up looking like:

xilem/masonry/src/widgets/checkbox.rs

Lines 290 to 296 in f0d7158

	let mut checkbox = checkbox.downcast::<Checkbox>();
	checkbox.set_checked(true);
	checkbox.set_text("The quick brown fox jumps over the lazy dog".into());
	let mut label = checkbox.label_mut();
	label.set_brush(PRIMARY_LIGHT);
	label.insert_style(StyleProperty::FontSize(20.0));