DU serialization uses property names

[pchalamet]

Use a field per field serialization for DU.

type Value =
    | None
    | Int of a:int * b:int
    | Float of float

This now leads to following serializations:

Value.None ==> { _t: "None" }

Value.Int (1, 2) ==> { _t: "Int", a: 1, b: 2 }

Value.Float 1.2 ==> { _t: "Float", Item: 1.2 }

❗ DU of record was and is still broken