Add ExpressibleByNilLiteral constraint to OptionalProtocol

[nkristek]

Hi, this is just a very small contribution regarding the OptionalProtocol and adds the ExpressibleByNilLiteral constraint to it. It is already implemented by Optional<Wrapped> and requires no further change. This makes it possible to initialise a value of type OptionalProtocol with nil instead of having to write .init(reconstructing: nil).

There also was a small typo in the documentation of the associatedType which I fixed as well.

Example

extension Property where Value: OptionalProtocol {
    func mapNil() -> Property<Value> {
        // now possible to write
        return Property(value: nil)
        // instead of
        return Property(value: .init(reconstructing: nil))
    }
}

Extending OptionalProtocol with the implementation of ExpressibleByNilLiteral is not possible due to a language constraint:

/// error: Extension of protocol 'OptionalProtocol' cannot have an inheritance clause
extension OptionalProtocol: ExpressibleByNilLiteral {
    
}

For now, as a workaround, one needs to append ExpressibleByNilLiteral to the generic constraint like so:

extension Property where Value: OptionalProtocol & ExpressibleByNilLiteral {
    func mapNil() -> Property<Value> {
        return Property(value: nil)
    }
}

Checklist

• Updated CHANGELOG.md.