Move Keyring and fingerprint to DcKey trait

This moves both the Keyring and the fingerprints to the DcKey trait,
unfortunately I was not able to disentangle these two changes.  The
Keyring now ensures only the right kind of key is added to it.

The keyring now uses the DcKey::load_self method rather than
re-implement the SQL to load keys from the database.  This vastly
simpliefies the use and fixes an error where a failed key load or
unconfigured would result in the message being treated as plain text
and benefits from the in-line key generation path.

For the fingerprint a new type representing it is introduced.  The aim
is to replace more fingerpring uses with this type as now there are
various string representations being passed around and converted
between.  The Display trait is used for the space-separated and
multiline format, which is perhaps not the most obvious but seems
right together with FromStr etc.

src/securejoin.rs

@@ -14,7 +14,7 @@ use crate::e2ee::*;
 use crate::error::{bail, Error};
 use crate::events::Event;
 use crate::headerdef::HeaderDef;
-use crate::key::{dc_normalize_fingerprint, DcKey, Key, SignedPublicKey};
+use crate::key::{dc_normalize_fingerprint, DcKey, SignedPublicKey};
 use crate::lot::LotState;
 use crate::message::Message;
 use crate::mimeparser::*;
@@ -142,7 +142,7 @@ pub async fn dc_get_securejoin_qr(context: &Context, group_chat_id: ChatId) -> O
 
 async fn get_self_fingerprint(context: &Context) -> Option<String> {
     match SignedPublicKey::load_self(context).await {
-        Ok(key) => Some(Key::from(key).fingerprint()),
+        Ok(key) => Some(key.fingerprint().hex()),
         Err(_) => {
             warn!(context, "get_self_fingerprint(): failed to load key");
             None