Define an order for allMembers traits

[Adam D Ruppe]

On Wednesday, 23 August 2023 at 13:53:30 UTC, Andrey Zherikov 
wrote:
> ```d
> struct Params
> {
>     @PositionalArgument(0)
>     string firstName;
>
>     @PositionalArgument(1)
>     string middleName;
>
>     @PositionalArgument(2)
>     string lastName;
> }
> ```

tbh I actually prefer this code to the alternative

> If `allMembers` guarantees the order to be the same as the 
> members are declared then the code above can be simplified to 
> this:

but yeah, what others say about the tuple thing is prolly the 
best thing by spec

tho in practice allMembers is in order but relying on that might 
lead to troube some random day