Change EventHandler to always pass by reference and kill return-value semantics.

tests/EventManagerTest.php

@@ -19,7 +19,7 @@ class EventManagerTestInvalidSubscriber
 
 class EventManagerTestSubscriber implements EventHandler
 {
-    public static function handleEvent(string $event, array $context) {}
+    public static function handleEvent(string $event, array &$context) {}
 }
 
 class EventManagerTest extends ModelTestCase

tests/FakeModule/Module.php

@@ -6,7 +6,7 @@ use LotGD\Core\Game;
 use LotGD\Core\Module as ModuleBase;
 
 class Module implements ModuleBase {
-    public static function handleEvent(string $event, array $context) {
+    public static function handleEvent(string $event, array &$context) {
         $context['foo'] = 'baz';
         return $context;
     }