- Also refactored internals to use weak/shared pointers

To prevent too much disruption to people who have our public projects as
part of their build or testing systems we are temporarily ignoring a
warning about using Xcode 15.

This allows real-time threads to join an audio workgroup on Apple platforms.

- added unit tests
- best performance timers used for each platform
- fixed an issue in which timer callbacks could drift

Xcode 14 has minimum deployment targets of macos 10.13 and iOS 11.