Sum Types - first draft

[Walter Bright]

https://github.com/WalterBright/documents/blob/96bca2f9f3520cf53ed5c4dec8e5e2d855e64e66/sumtype.md


I wrote that some time ago back in November 2022. The idea is to have a sumtypes 
proposal, followed by a match proposal.

Previous discussions:

https://www.digitalmars.com/d/archives/digitalmars/D/sumtypes_for_D_366242.html

https://www.digitalmars.com/d/archives/digitalmars/D/Sum_type_the_D_way_366389.html

https://www.digitalmars.com/d/archives/digitalmars/D/draft_proposal_for_Sum_Types_for_D_366307.html