Refactor form data submission

[domenic]

As a first step for WICG/webcomponents#187 (adding customizable form data for custom elements), we decided at TPAC 2016 that we should refactor constructing the form data set by having it call out to each element (probably also each input type), instead of having it be a big switch statement inlined into the algorithm. This will help us illuminate the general requirements for what custom elements will need.

[domenic]

Eh. I like the switch statement these days. Perhaps Stockholm syndrome. But we managed to do form-associated custom elements with it as well.