Code polish

- Fix some typos.
- Added more comments (and documentation) to the code.

src/types/introducer.rs

@@ -108,6 +108,7 @@ use std::fmt::{Display, Formatter};
 impl Display for Introducer {
 
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        // XXX: This is unsafe as ANSI control chars are not handled.
         let uids = self.cert.userids()
             .map(|u| String::from_utf8_lossy(u.userid().value()).to_string())
             .collect::<Vec<String>>()