Add a rough "multipart/form-data" stream parser

Summary:
Depends on D18699. Ref T13008. This only handles the basics (and doesn't stream files to disk yet) but provides a stream-oriented parser for multipart/form-data HTTP requests.

I'm going to integrate this into web workflows next and add coverage / fix bugs as I hit them.

Test Plan: Added basic unit tests, which pass.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13008

Differential Revision: https://secure.phabricator.com/D18700