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_when_done.wren

@@ -5,9 +5,9 @@ var a = new Fiber {
 // Run a through an intermediate fiber since it will get discarded and we need
 // to return to the main one after a completes.
 var b = new Fiber {
-  a.run
+  a.run()
   IO.print("nope")
 }
 
-b.call // expect: run
-a.run // expect runtime error: Cannot run a finished fiber.
+b.call() // expect: run
+a.run()  // expect runtime error: Cannot run a finished fiber.