Align Differentiable with Identifiable protocol

[Marcocanc]

Checklist

• Reviewed the README and documents.
• Searched existing issues for ensure not duplicated.

Description

Since the identity component of Differentiable is semantically identical to the new Identifiable protocol in Swift 5.1, I think it would be beneficial to align the two APIs.

Motivation and Context

This allows us to easily conform to Identifiable as well as Differentiable through a single implementation.

Proposed Solution

• Separate Equality (ContentEquatable) and Identification (ContentIdentifiable) component through the use of protocol composition for Differentiable
• Align the API of ContentIdentifiable to that of Identifiable

See #81 for implementation details

[ra1028]

Thank you for your proposal @Marcocanc

As you described, Identifiable and Differentiable have semantically the same properties.
However, Identifiable has OS restrictions, and I think there are few benefits to use it with breaking API.
How about implementing such a simple extension?

public extension Differentiable where Self: Identifiable {
    @inlinable
    var differenceIdentifier: ID {
        id
    }
}

[Marcocanc]

Thanks for the quick feedback. I understand that aligning the API might not be worth breaking existing code bases.

I have created a separate PR (#83) which introduces ContentIdentifiable and removes its dependency from ContentEquatable. Differentiable is now a composed protocol.
This will allow to work with the protocols on a more granular level and avoid any breaking APIs.

[ra1028]

Thanks @Marcocanc

The changes by #83 breaks the existing API, but agrees that it can be further clarified by separating identification and equality, so I'll allow this and include it in the next version 1.2.0.

Whether to change the naming differenceIdentifier to id will be discussed again when DifferenceKit grows to version 2.0.0.