- Add new shared subcrate satrs-shared to split off some shared
components not expected to change very often.
- Renmame `satrs-core` to `satrs`. It is expected that sat-rs will remain
the primary crate, so the core information is superfluous, and core also
implies stability, which will not be the case for some time.
- Added diagrams for sat-rs example for both structure and data flow.
- Added explanations for those diagrams as well.
- Some renaming: Use `Pool` instead of `Store` for pool components.
- General improvements for satrs-book.
