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.wren

@@ -1,14 +1,14 @@
 var fiber = new Fiber {
   IO.print("fiber 1")
-  Fiber.yield
+  Fiber.yield()
   IO.print("fiber 2")
-  Fiber.yield
+  Fiber.yield()
   IO.print("fiber 3")
 }
 
-var result = fiber.call // expect: fiber 1
-IO.print("main 1")  // expect: main 1
-result = fiber.call // expect: fiber 2
-IO.print("main 2")  // expect: main 2
-result = fiber.call // expect: fiber 3
-IO.print("main 3")  // expect: main 3
+var result = fiber.call() // expect: fiber 1
+IO.print("main 1")    // expect: main 1
+result = fiber.call() // expect: fiber 2
+IO.print("main 2")    // expect: main 2
+result = fiber.call() // expect: fiber 3
+IO.print("main 3")    // expect: main 3