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-mib/codegen/tests/basic.rs

@@ -1,8 +1,8 @@
 //! Basic check which just verifies that everything compiles
-use satrs_core::res_code::ResultU16;
 use satrs_mib::resultcode;
+use satrs_shared::res_code::ResultU16;
 
 #[resultcode]
-const _TEST_RESULT: ResultU16 = ResultU16::const_new(0, 1);
+const _TEST_RESULT: ResultU16 = ResultU16::new(0, 1);
 
 fn main() {}