[Feature] Discriminated unions (fast lookup)

[jakubwolny]

Motivation:
I have a union of around 50 types, from which some are very complex. In this scenario input assertion takes significant time to be evaluated. As I understand superstruct union - it iterates over all union members until there's match. All of these types have a common field "type" which could be used for faster lookup.

Idea:
When checking around I noticed "zod" has something exactly like this
https://github.com/colinhacks/zod#discriminated-unions

Is this a feature you'd welcome in superstruct?

[birtles]

I made something up over in #897 (comment) for this but I've never got around to polishing it up to be merged