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/ccsds.rs

@@ -1,6 +1,6 @@
-use satrs_core::pus::ReceivesEcssPusTc;
-use satrs_core::spacepackets::{CcsdsPacket, SpHeader};
-use satrs_core::tmtc::{CcsdsPacketHandler, ReceivesCcsdsTc};
+use satrs::pus::ReceivesEcssPusTc;
+use satrs::spacepackets::{CcsdsPacket, SpHeader};
+use satrs::tmtc::{CcsdsPacketHandler, ReceivesCcsdsTc};
 use satrs_example::config::PUS_APID;
 
 #[derive(Clone)]