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/yield_with_no_caller.wren

@@ -1,9 +1,9 @@
 var a = new Fiber {
-  Fiber.yield // expect runtime error: No fiber to yield to.
+  Fiber.yield() // expect runtime error: No fiber to yield to.
 }
 
 // Run a chain of fibers. Since none of them are called, they all get discarded
 // and there is no remaining caller.
-var b = new Fiber { a.run }
-var c = new Fiber { b.run }
-c.run
+var b = new Fiber { a.run() }
+var c = new Fiber { b.run() }
+c.run()