Making enum join variadic

["Nordlöw" via Digitalmars-d-learn]

> I'd be verbose. It's an uncommon operation, bound to surprise a 
> reader
> a bit.It's better to type a few more letters.

See update.

> I did not try to compile it, but what happens if the enum 
> elements
> have the same name ? The same min/max/values ?
>
> Like this:
>
> enum E1 { a, b, c }
>
> alias E111 = join!(E1, E1, E1);

This should give a better error message, than current mixin error.
I'll work on that.

> I would put it in std.typecons, since it's a type constructor

Ok.

Thx!