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