Capability to get children compile-time info.

This commit adds a feature of compile-time introspection of aggregate types' "children".

E.g.
```d
class RootOfSomeRuntimeRegisteringHierarchy
{
    struct _RTInfo(T)
    {
        static this()
        {
            writeln("Registering runtime info for ", T.stringof);
        }
    }
}

class A : RootOfSomeRuntimeRegisteringHierarchy {}
```

Will output:
```
Registering runtime info for RootOfSomeRuntimeRegisteringHierarchy
Registering runtime info for A
```