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/run_with_value_direct_reenter.wren

@@ -6,5 +6,5 @@ fiber = new Fiber {
   IO.print(2) // expect: 2
 }
 
-fiber.call
+fiber.call()
 IO.print(3) // expect: 3