Improve parsing of use statements

Summary:
The way that we parse use statements at the moment is a bit odd. For example, consider the following example:

<?php

use A\B\C;
use X\Y\Z;

This is currently parsed as:

After this change, it is instead parsed as:

Test Plan: Added a test case.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14518