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/src/lib.rs

@@ -0,0 +1,3 @@
+//! This library contains modules shared amongst various sat-rs framework crates.
+#![no_std]
+pub mod res_code;