use Maybe type for error handling (Fail case)

[unional]

NumericStruct.Add<NumericStruct.FromNumeric<A, Fail>, NumericStruct.FromNumeric<B, Fail>>
//              ^ fail as input not accepting `Fail`

Instead of:

type NumericStruct.Add<A extends NumericStruct, B extends NumericStruct> ...

Accept Maybe<NumericStruct> and propagate the error.

This will simplify many implementation.