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?

[Lukasa]

I think a lower-impact way of doing this would be to avoid renaming the file, and to put the typealiases in a file whose name ends in _boring.swift. That extension makes it clear to the tooling that keeps this in sync with CryptoKit that it's an extension for Crypto only, and so would ensure it's preserved over API syncs.

Does that work for you?