Implement extremely basic form submission

This is missing a lot of parts, so it doesn't really make any real-world form submission work yet. It provides a skeleton on which the missing bits can be filled in.

What works:
 - `<input>` elements except for `type=file`
 - GET/POST methods
 - URLencoded `enctype`s (default)
 - Submission via `<form>.submit()` only

Stuff that needs to be done for most simple real-world cases to work: 
 - [Working text input](https://github.com/servo/servo/pull/3585)
 - Click handlers for `<submit>`
 - Possibly `<textarea>` support
 - Support for the other two enctypes (#3649)

Todo:
 - Correctly implement [planned navigation](https://html.spec.whatwg.org/multipage/forms.html#planned-navigation) using `TrustedFormAddress`  (#3648)
 - [Correctly implement form owners.](https://github.com/servo/servo/issues/3553) Requires html5ever and some discussion of the spec.
 - `<input type=file>` support
 - Image submit support
 - Browsing contexts/targets
 - Support for non-`<input>` controls
 - Validation (?)
 - Dirname (?)