Tagging untagged union to a `Variant`

[paluh]

Explore idea of concrete helpers to do lifting of untagged union values into a Variant. I mean through directly passed decoders. Not sure if this makes any sens - it is first dump:

foreign import data :: Row Type -> Untagged

foreign import data tag :: Untagged r -> { | typeGuards? } -> Variant r

By typeGuards I mean row of functions like these: https://www.typescriptlang.org/docs/handbook/advanced-types.html#user-defined-type-guards