Allow to get first item of a collection

Summary:
This changes add to BaseCollection classes the following methods:

• first() will return the first item if it exists
• firstOr() will return the first item if it exists, or a default value if not.

Both are implemented by the WithCollection trait.

Test Plan: Tested in Zed

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3197