No need for version expression is a lie

[Dukc]

On Thursday, 24 August 2023 at 17:03:20 UTC, Walter Bright wrote:
> You wrote that this is in hundreds of places in druntime. 
> Great! If this technique wasn't used, I guarantee that any port 
> to AdrOS will miss more than a few, and it will be hell to 
> figure out what is going wrong. I guarantee it because it 
> always happened when defaults were used for unknown systems.

I understand this point, but it doesn't take out what Adam said. 
Sometimes you're coding for a platform where DRuntime isn't 
ported to, without intention to port it in whole.

I think we want to let the user to pick. By default, 
unimplemented DRuntime functions would cause a compilation 
failure, but that could be suppressed with `-version=Autostub`. 
Therefore, the versioning would be:

```D
else version(Autostub){}
else static assert(0, "Not implemented!");
```

Alternatively for less verbose syntax, `object.d` would have

```D
version(Autostub) enum autostubbing = true;
else enum autostubbing = false;
```

...making the versioning tail look like:

```D
else static assert(autostubbing, "Not implemented!");
```