Define an order for allMembers traits

[Paul Backus]

On Wednesday, 23 August 2023 at 12:32:58 UTC, Andrey Zherikov 
wrote:
> This will especially help in my use case where I need to know 
> the order in which struct members are declared. Right now I'm 
> using UDA with index to explicitly set an order:
> ```d
> struct S
> {
>   @UDA(1) int a;
>   @UDA(2) int b;
>   @UDA(3) string c;
>   @UDA(4) string d;
> }
> ```

For this use case, you should use `.tupleof` instead of 
`__traits(allMembers)`, which [is guaranteed by the spec to give 
the fields in declaration order][1].

[1]: https://dlang.org/spec/class.html#class_properties