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_value.wren

@@ -6,9 +6,9 @@ var fiber = new Fiber {
   IO.print("fiber 3")
 }
 
-var result = fiber.call // expect: fiber 1
+var result = fiber.call() // expect: fiber 1
 IO.print(result) // expect: yield 1
-result = fiber.call // expect: fiber 2
+result = fiber.call() // expect: fiber 2
 IO.print(result) // expect: yield 2
-result = fiber.call // expect: fiber 3
+result = fiber.call() // expect: fiber 3
 IO.print(result) // expect: null