Closures are not the only types of callable objects in PHP.
Specifically, any string referencing a valid function, any object with a __call(),
or any class with a __callStatic() can all be called.
Therefore, removed type hinting for Closures in places where a callable is expected.
(Unfortunately, the callable type-hint only comes in PHP 5.4.)
Change-Id: I6bff7e4a95716ef63aa7e07d3d9fef6d20eb65a6
