Consider splitting `Nothing<T>` into `None<T>` and `Nil<T>`

[amikheychik]

The key feature of the Maybe monad implementation is its strict typing. For example, the type of just().value is always T.

But the type of nothing().value and nil().value is null | undefined, even internally these two functions create Nothing with either undefined or null.

This can be improved by making the Nothing class abstract and introducing two concrete classes Nil and None with null and undefined values only.