Re-structure sat-rs

- 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.

satrs-shared/README.md

@@ -0,0 +1,5 @@
+satrs-shared
+=========
+
+Subcrate for some shared sat-rs framework components not expected to change very often.
+You can find more information on the [homepage](https://egit.irs.uni-stuttgart.de/rust/sat-rs).