Align with Identifiable, Add ContentIdentifiable

[Marcocanc]

Checklist

• All tests are passed.
• Added tests.
• Documented the code using Xcode markup.
• Searched existing pull requests 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.

Related Issue

#80

Motivation and Context

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

Impact on Existing Code

This will break conformance to Differentiable in existing imlpementations, unless implicit conformance through Hashable/Equatable was used.

Implementation Details

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

[ra1028]

Merge #83 instead of here.