Allow empty argument list methods.

- Compile them as calls and definitions.
- Use them for call(), clear(), run(), try(), and yield().
- Update the docs.

test/fiber/is_done_after_error.wren

@@ -2,5 +2,5 @@ var fiber = new Fiber {
   "s".unknown
 }
 
-fiber.try
+fiber.try()
 IO.print(fiber.isDone) // expect: true