Add derive macro to faciliate elements creation

[nixonyh]

• Splits the Element trait into ElementNew (construction), ElementChildren (child iteration), and ElementBuild (layout/render)
• Adds #[derive(Element)] which generates the boilerplate for ElementSlot, ElementNew, and ElementChildren.
• Adds #[derive(ElementSlot)] for type registration only

Example

#[derive(Element, Default, Clone)]
struct Vertical {
    #[children]
    children: Vec<ElementId>,
}

impl ElementBuild for Vertical {
    fn build(&self, ..) -> Size { .. }
}

• #[children] works on anything that implements IntoIterator<&ElementId>
• Struct-level overrides via #[element(new = fn, children = fn)] for custom construction or child iteration.

[nixonyh]

Probs want to call this in Fynix::new() instead so users don't need to be concerned with this thing.