Add inequality operator ( != ) for digest and DataProtocol comparison

[joey-gm]

Add an inequality comparison operator to determine whether a digest is not equal to a collection of contiguous bytes.

Checklist

• I've run tests to see all new and existing tests pass
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

Motivation:

The equal ( == ) and not equal ( != ) operators are available for comparing two digests. However, only the equal operator is available for the comparison between a digest and a collection of contiguous bytes.

This modification adds the missing inequality operator ( != ) for digest and DataProtocol comparison.

Modifications:

The following static method has been added to Digest.swift as an extension to Digest.

public static func != <D: DataProtocol>(lhs: Self, rhs: D) -> Bool

A corresponding test case has also been added to DigestsTests.swift.

Result:

Instead of checking if !(digest == someDataBytes) {...}, we can now write if digest != someDataBytes {...}.

[Lukasa]

Thanks for this! As with several other PRs open on this repo, any change to the public API surface requires synchronization with CryptoKit in Apple's SDKs. While this is a reasonable change, I'm going to defer merging it for now.