Unwind HtmlFormElement and improve the FormData API

[TheSpyder]

There are two main problems with Webapi__Dom__HtmlFormElement:

• It uses the Impl pattern but is never included by other files
• It defines a data function to create a FormData object, which is both a weird name for the constructor and a weird place to put it

The first is an easy fix; just move all the bindings to the end of the file and delete the Impl module.

For the second, just delete data and re-do #108 (which was reverted after I discovered this).