#Nil_Is_Success Move Nil under the Success Interface to make clear that Nil is a succes

[RamAnvesh]

Today, the Errable class hierarchy is three independent children: Success, Failure and Nil. This can be confusing. When a computation returns a Nil, it is considered success, but with no result.

We need to move Nil under Success:

sealed interface Errable permits Success, Failure
sealed interface Success permits Value, Nil

final class Success<T> implements Errable<T>
final class Value<T> implements Success<T>
final class Nil<T> implements Success<Void>
final class Failure<T> implements Errable<Void>