Add support for nullable parameter types

Summary: Ref T4334. Adds support for nullable parameter types, which are a thing as of PHP 7.1.

Test Plan: Added unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, wjiang, thaiphv, Firehed

Maniphest Tasks: T4334

Differential Revision: https://secure.phabricator.com/D18770