__metadata should register function that returns type instead of literal type

Imagine case with circular dependencies ``` class Car { @Field owner: Person // !!! Error: Person is not defined. } class Person { @Field car: Car; } // car has owner, owner has car ``` Typescript metadata would be emitted here like eg. `__metadata('design:type', Person)`. As `Person` is injected for the first time before `Person` class is initialized, it will result with `ReferenceError` saying `Person is not defined`. If it'd emit metadata like: ```__metadata('design:type', () => Person)``` it'd be fine. Later on, when using Reflect.metadata, it would also need to call meta function instead of just returning the type. If you think it's good idea, do you have any suggestions about starting point for PR that would implement this change?