Sign artifacts after compilation

If compilation succeeds, pass over all the artifacts we generated and
produce an OdsignInfo file with all the names and digests and a
signature of it using the CompOS key.

Various refactorings along the way to make it eaiser:
- Extract the odrefresh ExitCode enum for reuse
- Extracted a Signer, distinct from an ArtifactSigner
- Defined OdrefreshContext to stop the linter complaining about a
  function with >7 arguments.

Also added more debugging to help fix my various silly mistakes.

Bug: 161471326
Test: composd_cmd forced-odrefresh -> signature file produced
Change-Id: Ia6a6abd9e28158f4767e7752727635087f9bb566
diff --git a/compos/common/Android.bp b/compos/common/Android.bp
index 5893fd6..7c61d94 100644
--- a/compos/common/Android.bp
+++ b/compos/common/Android.bp
@@ -14,8 +14,10 @@
         "libbinder_rpc_unstable_bindgen",
         "libbinder_rs",
         "liblog_rust",
+        "libnum_traits",
         "librustutils",
     ],
+    proc_macros: ["libnum_derive"],
     shared_libs: [
         "libbinder_rpc_unstable",
     ],