Use nightly rustfmt features to improve code style (#734)

* format_code_in_doc_comments

* imports_granularity

* group_imports

* Add ci job

src/transport/smtp/authentication.rs

@@ -1,8 +1,9 @@
 //! Provides limited SASL authentication mechanisms
 
-use crate::transport::smtp::error::{self, Error};
 use std::fmt::{self, Debug, Display, Formatter};
 
+use crate::transport::smtp::error::{self, Error};
+
 /// Accepted authentication mechanisms
 ///
 /// Trying LOGIN last as it is deprecated.