Make clippy happy

- Applied many of the `clippy` suggestions.

src/types/introducer.rs

@@ -64,7 +64,7 @@ impl Display for Introducer {
             .map(|u| String::from_utf8_lossy(u.userid().value()).to_string())
             .collect::<Vec<String>>()
             .join(", ");
-        write!(f, "Introducer: {}\n", self.cert.fingerprint())?;
+        writeln!(f, "Introducer: {}", self.cert.fingerprint())?;
         write!(f, "  UserIDs: {}", uids)?;
         Ok(())
     }