Basic persistence

Basic CRUD for models:

- [x] `::find`
- [x] `::[]` (bare initialization)
- [x] `#save`
- [x] `#destroy`
- [x] Atomic collection mutation operations

Does not encompass:

* More complex lookups (e.g. `find_by_*`)
* Column-level load configurations
* Nested models
* Reference lookups
* Validations, lifecycle hooks, etc.