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/abort.wren

@@ -2,6 +2,6 @@ var fiber = new Fiber {
   Fiber.abort("Error message.")
 }
 
-IO.print(fiber.try) // expect: Error message.
+IO.print(fiber.try()) // expect: Error message.
 IO.print(fiber.isDone) // expect: true
 IO.print(fiber.error) // expect: Error message.