But they use the plist format for serialization (as plists theirself use the XML format).

https://github.com/apple/swift-corelibs-foundation/blob/master/Foundation/NSKeyedArchiver.swift

Direct support of keyed archives would be better to implement in third-party package. But we can provide the support for low-level operations.

For distinguishing UIDs from integers and for being able to create plist files containing UIDs we need a special purposed class plist.UID. It will be a simpler wrapper around int with few methods: __index__(), __repr__(), __reduce__().