xmpp-parsers: Replace bind::Required with an #[xml(flag)]

This makes the code simpler to read.

parsers/src/bind.rs

@@ -15,15 +15,10 @@ use jid::{FullJid, Jid};
 #[xml(namespace = ns::BIND, name = "bind")]
 pub struct BindFeature {
     /// Present if bind is required.
-    #[xml(child(default))]
-    required: Option<Required>,
+    #[xml(flag)]
+    required: bool,
 }
 
-/// Notes that bind is required.
-#[derive(FromXml, AsXml, Debug, Clone, PartialEq)]
-#[xml(namespace = ns::BIND, name = "required")]
-pub struct Required;
-
 /// The request for resource binding, which is the process by which a client
 /// can obtain a full JID and start exchanging on the XMPP network.
 ///
@@ -84,7 +79,6 @@ mod tests {
     #[test]
     fn test_size() {
         assert_size!(BindFeature, 1);
-        assert_size!(Required, 0);
         assert_size!(BindQuery, 12);
         assert_size!(BindResponse, 16);
     }
@@ -93,7 +87,6 @@ mod tests {
     #[test]
     fn test_size() {
         assert_size!(BindFeature, 1);
-        assert_size!(Required, 0);
         assert_size!(BindQuery, 24);
         assert_size!(BindResponse, 32);
     }

parsers/src/starttls.rs

@@ -60,6 +60,7 @@ mod tests {
     fn test_size() {
         assert_size!(Request, 0);
         assert_size!(Proceed, 0);
+        assert_size!(Failure, 0);
         assert_size!(StartTls, 1);
         assert_size!(Nonza, 1);
     }