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/lot.rs

@@ -1,5 +1,7 @@
 use deltachat_derive::{FromSql, ToSql};
 
+use crate::key::Fingerprint;
+
 /// An object containing a set of values.
 /// The meaning of the values is defined by the function returning the object.
 /// Lot objects are created
@@ -14,7 +16,7 @@ pub struct Lot {
     pub(crate) timestamp: i64,
     pub(crate) state: LotState,
     pub(crate) id: u32,
-    pub(crate) fingerprint: Option<String>,
+    pub(crate) fingerprint: Option<Fingerprint>,
     pub(crate) invitenumber: Option<String>,
     pub(crate) auth: Option<String>,
 }