Typealias CryptoKitError

[0xTim]

Provides a typealias for CryptoKitError to hide the implementation detail of CryptoKit. Resolves #274

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

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:

CryptoKit is an implementation detail on Apple platforms - this makes it clearer for users coming from other platforms

Modifications:

Added a type alias and updated documentation

Result:

[After your change, what will change.]

[0xTim]

@Lukasa is there anything more that needs to be done on this?

[0xTim]

@Lukasa ok updated the PR to avoid any filename changes and kept the renames to just the API docs (as opposed to changing the code to throw a CryptoError.

I think the CryptoExtras code should be updated to remove any references to CryptoKit but that can be done in a separate PR

[Lukasa]

Ah shoot, can you rebuild the CMake pieces? There's a script.