object.factory with template classes for serializing subclasses automatically

[timotheecour]

> Also, now that I think about it, why couldn't you do this? 
> (it's equivalent):
>
> auto serialize(T)(T a) {
>     auto c = cast(SerializerBase) new Serializer!T;
>     return c.serialize(a);
> }

that won't work with my example:
class A{}
class B:A{int x;}
A a=new B;
auto c=serialize(a); => T is A, but we want B so that 
serialization includes field x.