Add Sequence conformance to AES._CBC.IV (#389)

[wnagrodzki]

Add Sequence conformance to AES._CBC.IV where Element is UInt8.

Checklist

• I've run tests to see all new and existing tests pass

PKCS8DERRepresentationTests do not compile on master branch with Error 'pkcs8DERRepresentation' is only available in iOS 14.0 or newer. All the other tests pass after my changes.

• 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

If you've made changes to gyb files

• I've run .script/generate_boilerplate_files_with_gyb and included updated generated files in a commit of this pull request

Motivation:

No convenient API for converting AES._CBC.IV into Data.

Modifications:

Added Sequence conformance to AES._CBC.IV similarly how it is done for AES.GCM._SIV.Nonce.

Result:

It will be possible to create Data instance with AES._CBC.IV passed as init argument.

[Lukasa]

Thanks for this! Can you also add a test-case that uses this API so we can make sure it works as expected?

[Lukasa]

Let's erase this to some IteratorProtocol, and make the method @inlinable. That should give us the same performance characteristics as the existing code, but allow us to change the iterator in a future release if we feel like it.

[Lukasa]

Very nice, thanks!