Explicit no-change render node #262

(This overlaps with #217 and similar.)

I've only just found Vecty, and maybe I'm getting ahead of myself, but I'm wondering about the amount of work involved in creating and comparing the render trees for big pages that don't significantly change between renders.

E.g. if I have 1000 elements in my page, there's a good chance that 990 don't change ever, and also that these elements are grouped together.

Is there a method (or would a change be considered) of allowing explicit no-change render nodes, maybe something like:

func NewPageView() *PageView {
	p := &PageView{}

	p.header = memoise(p.renderHeader())
	p.child = &ChildView{}
	p.footer = memoise(p.renderFooter())

	return p
}

func (p *PageView) Render() vecty.ComponentOrHTML {
	return elem.Body(
		p.header,
		child,
		p.footer,
	)
}

The explicit storage of the subtree would avoid accidental changes, and also shortcut the tree comparison by allowing a version id to be embedded in the real DOM.

Thoughts? Thanks!