Modernize Tracks Apple

Taking a page from the work @wzieba and @ParaskP7 have been doing on the [Android counterpart](https://github.com/Automattic/Automattic-Tracks-Android/), such as [decoupling ExPlat from FluxC](https://github.com/Automattic/Automattic-Tracks-Android/pull/244), here are a few ideas of things we could do to give Apple developers more capabilities via Tracks.

This is a bit vague because I haven't done any dedicated research into feasibility and current architecture.

- Investigate whether it's possible to decouple events from experiments, and if so define separate packages, in particular to decouple events/crash logging from experiments.
- Remove CoreData in favor of a simpler and more lightweight storage layer.
- Drop the Objective-C code to simply package structure and, consequently, maintenance.

Start by reviewing the current Automattic-Tracks-iOS architecture and the Android counterpart, especially the referenced ExPlat/FluxC decoupling work. Assess the feasibility of separating events, crash logging, and experiments, replacing CoreData, and dropping Objective-C; done should be a concrete, scoped plan rather than an implementation assembled from this issue alone.

Written by the indexing model from the issue text.