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.wren

@@ -3,7 +3,7 @@ var fiber = new Fiber {
 }
 
 IO.print("before") // expect: before
-fiber.run          // expect: fiber
+fiber.run()        // expect: fiber
 
 // This does not get run since we exit when the run fiber completes.
 IO.print("nope")