draft proposal for Sum Types for D

[Per Nordlöw]

On Tuesday, 29 November 2022 at 06:26:20 UTC, Walter Bright wrote:
> Go ahead, Make My Day! Destroy!
>
> https://github.com/WalterBright/DIPs/blob/sumtypes/DIPs/1NNN-(wgb).md

DIP is missing

- Canonicalization of `sumtype` members. Specifically:
   - Independence of order of members
     `sumtype S {T,U}` should be the same as `sumtype S {U,T}`.
   - Deduplication of members either as a compiler error or as a 
silent automatic removal of duplicate types. Moreover, what about
     `sumtype S {a T, b U}` vs `sumtype S {a U, b T}` and
     `sumtype S {a T, b U}` vs `sumtype S {b T, a U}`?

For reference see how `mir.algebraic.Variant` canonicalizes 
template type parameters using the trait `TypeSet` at 
https://github.com/libmir/mir-core/blob/master/source/mir/algebraic.d#L447.