feat(builder): Improve Message representation

* Remove bytes dependency and rely directly on bytes vec
* Allow encoding non-utf-8 strings
* Use Vec<u8> instead of Display for email formatting

examples/smtp.rs

@@ -16,7 +16,7 @@ fn main() {
     // Open a local connection on port 25
     let mut mailer = SmtpClient::new_unencrypted_localhost().unwrap().transport();
     // Send the email
-    let result = mailer.send(email);
+    let result = mailer.send(&email);
 
     if result.is_ok() {
         println!("Email sent");

examples/smtp_gmail.rs

@@ -23,7 +23,7 @@ fn main() {
         .transport();
 
     // Send the email
-    let result = mailer.send(email);
+    let result = mailer.send(&email);
 
     if result.is_ok() {
         println!("Email sent");