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/events.rs

@@ -1,6 +1,6 @@
 use std::sync::mpsc::{self, SendError};
 
-use satrs_core::{
+use satrs::{
     event_man::{
         EventManager, EventManagerWithMpscQueue, MpscEventReceiver, MpscEventU32SendProvider,
         SendEventProvider,