Make clippy happy

- Applied many of the `clippy` suggestions.

src/types/husk_context.rs

@@ -59,7 +59,7 @@ impl<'hc> HuskContext<'hc> {
 
     pub async fn add_recipient(&mut self, rcpt: String) {
 
-        let certs = crypto::get_certificates(&self, rcpt.as_str()).await;
+        let certs = crypto::get_certificates(self, rcpt.as_str()).await;
 
         self.mail.add_recipient(rcpt, certs)
     }

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(())
     }

src/types/mail_context.rs

@@ -66,6 +66,13 @@ impl<'mc> MailContext<'mc> {
 
 }
 
+/// Default Mailcontext
+impl<'mc> Default for MailContext<'mc> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl From<&MailContext<'_>> for ProtectionPossibility {
     fn from(mail_context: &MailContext<'_>) -> ProtectionPossibility {
         let mut can_encrypt = false;

src/types/recipient.rs

@@ -17,8 +17,8 @@ pub struct Recipient<'r> {
 
 impl From<String> for Recipient<'_> {
     fn from(email: String) -> Self {
-        Recipient { 
-            email: email, 
+        Recipient {
+            email,
             certs: Vec::new()
         }
     }
@@ -40,7 +40,7 @@ impl Recipient<'_> {
     /// check if this Recipient can encrypt.
     // XXX: currently this check is naive
     pub fn can_encrypt(&self) -> bool {
-        if self.certs.len() == 0 { 
+        if self.certs.is_empty() {
             return false;
         }