Polish the code

- Reject user ids containing ANSI control characters when creating a new
  signing key.
- Prevent adding certificates twice when querying online sources if they
  contain multiple encryption keys.
- Dedup introducers if there are multiple authenticating paths.
- When listing locals, skip the signing key.
- Check if a mail can be encrypted at all due to available certificates.
  If not tell the MTA that the processing is done.
- Make `Introducer` comparable (`PartialEq`, `Eq` and `Hash`).
- minor polishing

src/commands/locals.rs

@@ -20,11 +20,12 @@ pub async fn dispatch(cmd: LocalsCommand, config: HuskConfigContainer) -> Result
         LocalsSubcommand::List => {
             let context = HuskContext::new(&config.into())?;
 
-            println!("list locals");
+            // XXX: sort the output
             for local in crypto::get_locals(
                 &context.cert_store,
                 &context.policy,
-                context.local_trust_root.fingerprint()) {
+                context.local_trust_root.fingerprint(),
+                context.signing_key) {
 
                 println!("{}", local);
             }