Document user-defined sum types in `docs/design`

[geoffromer]

trafficstars

The "choice types" portion of p0157 is now documented in docs/design, but the design for user-defined sum types exists only in the proposal document. That's a problem because it's not very discoverable, and it can't be kept up to date since proposal docs are "frozen" (for example, it uses the old Type: value binding syntax instead of value: Type).