Change from using Option to std::iter::once when chaining together Form Iterators.

[mitchmindtree]

I've used Option throughout the widget draw methods to chain together Form Iterators (in order to avoid allocations), however I think the more idiomatic way would be to use std::iter::once (didn't see it until today).

For example, the line here

Some(circle_form).into_iter().chain(Some(text_form))

would become

once(circle_form).chain(once(text_form))