Add a "deprecated" attribute that generates #[deprecated] in rust source.

[lzrd]

It is probably necessary to use some #[allow(deprecated)] in the generated code so that a well behaved user is not getting spurious warnings. How do we eventually evict a deprecated entry and maintain the serialization that has been established for the remaining entries?

diff --git a/idl/sprot.idol b/idl/sprot.idol
index 0a0e1dd1..21921eaa 100644
--- a/idl/sprot.idol
+++ b/idl/sprot.idol
@@ -22,7 +22,8 @@ Interface(
idempotent: true,
),
"rot_state": (

•        doc: "Return boot-time info about the RoT (deprecated)",
  

•       deprecated: true,
  

•       doc: "Return boot-time info about the RoT",
         reply: Result(
             ok: "RotState",
             err: Complex("SprotError"),