version as an expression?

[ryuukk_]

Currently, i am doing this,( wich sucks

```D
version(Windows)
     alias socket_t = size_t;
else
     alias socket_t = int;
```

I'd prefer to be able to do:

```D
alias socket_t = version(Windows) size_t else int;
```

And better, if we had pattern matching and some builtin 
information available at compile time:


```D
alias socket_t = switch (builtin.target.os) {
     .windows => size_t,
     else     => int
};
```


Why i hate the current way of doing it, but i feel it'll be 
available in a distant future unfortunately:


- nothing will tell me if i forgot to implement an alias for a 
target until i compile for that target
- i repeat the name of the alias N times