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.wren

@@ -4,7 +4,7 @@ var fiber = new Fiber {
   IO.print("after")
 }
 
-IO.print(fiber.try)
+IO.print(fiber.try())
 // expect: before
 // expect: Bool does not implement 'unknownMethod'.
 IO.print("after try") // expect: after try