interface requirements : new lang feature [1/2]

interface requirements : new lang feature [1/2]

Summary: This is the logical extension of trait requirements necessary
 to capture the "marker interface" pattern, wherein an interface is
 implicitly tied to a type hierarachy as with the following code:

   class Super {
      ...
      if ($this instanceof IMarker) {
        $this->methodOfMarker();
      }
      ...
   }
   ...
   function marked(IMarker $inst) {
     $inst->methodOfSuper();
   }
   ...
   interface IMarker {
     ...
     // new syntax to indicate that IMarker is only for children of Super
     require extends Super;
     ...
   }

 This diff ensures that the syntax is supported in the parser, but
 does not enforce the requirements (left to next diff).

Reviewed By: @paroski

Differential Revision: D1387181