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/src/hal/std/mod.rs

@@ -0,0 +1,6 @@
+//! Helper modules intended to be used on systems with a full [std] runtime.
+pub mod tcp_server;
+pub mod udp_server;
+
+mod tcp_cobs_server;
+mod tcp_spacepackets_server;