Eliminate redundant conversions to Option and usage of filter_map. Use flat_map directly instead.

Documentation:
- https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.filter_map
- https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.flat_map

Additional Changes:
- Use point-less style and pass Email::try_from function directly to
  flat_map.
  flat_map passes uid to Email::try_from implicitly. That's why it's
  called point-less style. IMO it looks nicer!

database/src/lib.rs

@@ -296,7 +296,7 @@ pub trait Database<'a>: Sync + Send {
                     .userids()
                     .filter(|binding| !is_status_revoked(binding.revocation_status(&POLICY, None)))
                     .map(|binding| binding.userid())
-                    .filter_map(|uid| Email::try_from(uid).ok())
+                    .flat_map(Email::try_from)
                     .any(|unrevoked_email| &unrevoked_email == *email);
                 !has_unrevoked_userid
             })
@@ -476,7 +476,7 @@ pub trait Database<'a>: Sync + Send {
             .unwrap_or_default();
         let published_emails_old: Vec<Email> = published_uids_old
             .iter()
-            .filter_map(|uid| Email::try_from(uid).ok())
+            .flat_map(Email::try_from)
             .collect();
 
         // println!("publishing: {:?}", &uid_new);