Improve the OOP ABI

[Richard (Rikki) Andrew Cattermole]

On 29/09/2023 1:42 AM, deadalnix wrote:
> 1/ Downcast to final classes.

> ```d
> auto foo(A a) {
>      return typeid(a) is typeid(B) ? a : null;
> }
> ```

TypeInfo isn't guaranteed to be unique to a process when dealing with 
shared libraries. There is a strong chance it'll be duplicative.

So this won't work.

> 2/ Inline ClassInfo with the vtbl.

ClassInfo is an alias to TypeInfo_Class now. This indirection shouldn't 
exist.

https://github.com/dlang/dmd/blob/a6285c26c0e04b9b3b1d40ae1c3200c834df1480/druntime/src/object.d#L1737