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/call_when_done.wren

@@ -2,5 +2,5 @@ var fiber = new Fiber {
   IO.print("call")
 }
 
-fiber.call // expect: call
-fiber.call // expect runtime error: Cannot call a finished fiber.
+fiber.call() // expect: call
+fiber.call() // expect runtime error: Cannot call a finished fiber.