[lint] Add HLint-based Haskell linter

Summary:
This adds a lint engine for hlint, which is the standard and most general Haskell lint tool around these days.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

Test Plan: Install hlint, and run arc unit.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Projects: Arcanist

Differential Revision: https://secure.phabricator.com/D10250