Ensure ComponentOrHTML satisfies MarkupOrChild. (#162)
Ensure ComponentOrHTML satisfies MarkupOrChild. (#162)

This avoids some awkwardness, and API breakage for users returning
ComponentOrHTML from funcs as children in element constructors.

See https://github.com/gopherjs/vecty/pull/162#discussion_r148941678 for an example of such a case.
gopherjs/vecty