Move module functions to type methods

This moves the module-level lookup and creation functions to the
types, which make the naming more consistent.  Now the lookup_* get_*
and create_* functions all behave similarly.

Peraps even more important the API of the lookup now allows
distinguishing failure from not found.  This in turn is important to
be able to remove reliance on a ChatId with a 0 or "unset" value.  The
locations where this ChatId(0) is still used is in database queries
which should be solved in an independed commit.

src/securejoin/bobstate.rs

@@ -185,7 +185,7 @@ impl BobState {
         context: &Context,
         invite: QrInvite,
     ) -> Result<(Self, BobHandshakeStage), JoinError> {
-        let chat_id = chat::create_by_contact_id(context, invite.contact_id())
+        let chat_id = ChatId::create_for_contact(context, invite.contact_id())
             .await
             .map_err(JoinError::UnknownContact)?;
         if fingerprint_equals_sender(context, invite.fingerprint(), chat_id).await? {