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/pus/mode.rs

@@ -0,0 +1,16 @@
+use num_enum::{IntoPrimitive, TryFromPrimitive};
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone, IntoPrimitive, TryFromPrimitive)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[repr(u8)]
+pub enum Subservice {
+    TcSetMode = 1,
+    TcReadMode = 3,
+    TcAnnounceMode = 4,
+    TcAnnounceModeRecursive = 5,
+    TmModeReply = 6,
+    TmCantReachMode = 7,
+    TmWrongModeReply = 8,
+}