Use the Fingerprint type to handle fingerprints

This uses the Fingerprint type more consistenly when handling
fingerprits rather then have various string representations passed
around and sometimes converted back and forth with slight differences
in strictness.

It fixes an important bug in the existing, but until now unused,
parsing behaviour of Fingerprint.  It also adds a default length check
on the fingerprint as that was checked in some existing places.

Fially generating keys is no longer expensive, so let's not ignore
these tests.

src/mimeparser.rs

@@ -17,6 +17,7 @@ use crate::e2ee;
 use crate::error::{bail, Result};
 use crate::events::Event;
 use crate::headerdef::{HeaderDef, HeaderDefMap};
+use crate::key::Fingerprint;
 use crate::location;
 use crate::message;
 use crate::param::*;
@@ -44,7 +45,7 @@ pub struct MimeMessage {
     pub from: Vec<SingleInfo>,
     pub chat_disposition_notification_to: Option<SingleInfo>,
     pub decrypting_failed: bool,
-    pub signatures: HashSet<String>,
+    pub signatures: HashSet<Fingerprint>,
     pub gossipped_addr: HashSet<String>,
     pub is_forwarded: bool,
     pub is_system_message: SystemMessage,