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/is_done.wren

@@ -1,11 +1,11 @@
 var fiber = new Fiber {
   IO.print("1")
-  Fiber.yield
+  Fiber.yield()
   IO.print("2")
 }
 
 IO.print(fiber.isDone)  // expect: false
-fiber.call              // expect: 1
+fiber.call()            // expect: 1
 IO.print(fiber.isDone)  // expect: false
-fiber.call              // expect: 2
+fiber.call()            // expect: 2
 IO.print(fiber.isDone)  // expect: true