Support internally tagged non-nullary sums

[samhh]

Borrowing terminology from Serde: https://serde.rs/enum-representations.html#internally-tagged

For any given tag and any arbitrary sibling data:

Sum.Member<'Sun'> | Sum.Member<'Rain', { value: number }>

<->

{ tag: 'Sun' } | { tag: 'Rain', value: number }