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_direct_reenter.wren

@@ -2,9 +2,9 @@ var fiber
 
 fiber = new Fiber {
   IO.print(1) // expect: 1
-  fiber.run
+  fiber.run()
   IO.print(2) // expect: 2
 }
 
-fiber.call
+fiber.call()
 IO.print(3) // expect: 3