Warn users if using abnormally long tree depth

[Archmonger]

If someone has an idom.html tree that spans >= 5 levels deep, we should probably generate a warning that they should splitting it apart into components.

For example

html.div(
    html.div(
        html.div(
            html.div(
                html.div(
                    html.div("Hello World"),
                ),
            ),
        ),
    ),
)