[core] support embeddable/embedded entities

[CrystalMethod]

In some cases it makes sense to embed entities in some other entity.

@Embeddable
interface Address {
  ...
}

@Label // any other "entity" annotation
interface Person {
  ...
  @Embedded
  Address getAddress();
}

invalid:

xoManager.currentTransaction().begin();
Address a = xoManager.create(Address.class);
Person p = xoManager.create(Person.class);
p.setName("..");
xoManager.currentTransaction().commit(); // a will not have been persisted

valid:

xoManager.currentTransaction().begin();
Address a = xoManager.create(Address.class);
Person p = xoManager.create(Person.class);
p.setName("..");
p.setAddress(a);
xoManager.currentTransaction().commit();