policy-lang: consider nullable vs optional

[jdygert-spok]

Currently, optional types cannot be nested, so it behaves more like a nullable. Perhaps we should lean into this and make it more like e.g. C#'s nullable types, where it is like an extra value of the type rather than a sum type. Alternatively, we could make it nestable.

Nullable bool: true | false | null
Optional bool: Some(true) | Some(false) | None

I think the main practical difference would be allowing coercion into nullable, like let x: nullable int = 42