Add a linter rule for interface method bodies

Summary:
interface methods cannot contain a body. This construct will cause a fatal error:

PHP Fatal error:  Interface function X::Y() cannot contain body in /home/josh/workspace/github.com/phacility/arcanist/test.php on line 4

Test Plan: Added unit tests.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14561