a partial function mapping tag to constructor function, like
Attributes
Returns
a FromData instance
Example
enum Adt:
case A
case B(b: Boolean)
case C(a: Adt, b: Adt)
given FromData[Adt] = FromData.deriveEnum[Adt] {
case 0 => _ => Adt.A
case 1 => FromData.deriveConstructor[Adt.B]
case 2 => FromData.deriveConstructor[Adt.C]
}