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/try_direct_reenter.wren

@@ -1,7 +1,7 @@
 var fiber
 
 fiber = new Fiber {
-  fiber.try // expect runtime error: Fiber has already been called.
+  fiber.try() // expect runtime error: Fiber has already been called.
 }
 
-fiber.call
+fiber.call()