Check if array values are instances of something

Summary:
The main motivation for this change is that I sometimes dig down to some code and I wonder what's the type of a passed array.
It also promotes strong typing.

Test Plan:
Display diff with invalid Remarkup rule.
Fix it.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2042