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-example/src/requests.rs

@@ -1,7 +1,7 @@
 use derive_new::new;
-use satrs_core::hk::HkRequest;
-use satrs_core::mode::ModeRequest;
-use satrs_core::pus::verification::{TcStateAccepted, VerificationToken};
+use satrs::hk::HkRequest;
+use satrs::mode::ModeRequest;
+use satrs::pus::verification::{TcStateAccepted, VerificationToken};
 use satrs_example::TargetIdWithApid;
 
 #[allow(dead_code)]