Impl `ObjectLike` for `Either`

Since that trait is sealed, it would be nice and convenient to add an impl for it to `Either` if both sides implement it. That way, you could have a function that receives `Either<Table, AnyUserData>` and just use it normally, with a very high level API.

Later, you could implement `Value::(as/is)_object_like()` methods.

I would love to contribute this. What do you think?
