xmpp_parsers::jingle: add ReasonElement::other

The Jingle XEP states [1] that the `<reason/>` element may contain up
to one element which further qualifies the error condition:

> The <reason/> element MAY contain an element qualified by some other
> namespace that provides more detailed machine-readable information
> about the reason for the action.

The schema agrees:

> ```
>   <xs:complexType name='reasonElementType'>
>     <xs:sequence>
>       <xs:choice>
>         <!-- … omitted … -->
>       </xs:choice>
>       <!-- … omitted … -->
>       <xs:any namespace='##other' minOccurs='0' maxOccurs='1'/>
>     </xs:sequence>
>   </xs:complexType>
> ```

   [1]: https://xmpp.org/extensions/xep-0166.html#def-reason

parsers/src/jingle.rs

@@ -468,6 +468,11 @@ pub struct ReasonElement {
 
     /// A human-readable description of this reason.
     pub text: Option<String>,
+
+    /// A machine-readable extension of the reason.
+    // using a box here to save a significant amount of bytes in the common
+    // (absent) case.
+    pub other: Option<Box<Element>>,
 }
 
 impl fmt::Display for ReasonElement {
@@ -488,6 +493,7 @@ impl TryFrom<Element> for ReasonElement {
         check_no_attributes!(elem, "reason");
         let mut reason = None;
         let mut text = None;
+        let mut other = None;
         for child in elem.children() {
             if child.is("text", ns::JINGLE) {
                 check_no_children!(child, "text");
@@ -505,12 +511,18 @@ impl TryFrom<Element> for ReasonElement {
                 check_no_children!(child, "reason");
                 check_no_attributes!(child, "reason");
                 reason = Some(child.name().parse()?);
+            } else if other.is_none() {
+                other = Some(Box::new(child.clone()));
             } else {
                 return Err(Error::ParseError("Reason contains a foreign element."));
             }
         }
         let reason = reason.ok_or(Error::ParseError("Reason doesn’t contain a valid reason."))?;
-        Ok(ReasonElement { reason, text })
+        Ok(ReasonElement {
+            reason,
+            text,
+            other,
+        })
     }
 }
 
@@ -524,6 +536,7 @@ impl From<ReasonElement> for Element {
                     .into_iter()
                     .map(|text| Element::builder("text", ns::JINGLE).append(text)),
             )
+            .append_all(reason.other.into_iter().map(|other| *other))
             .build()
     }
 }
@@ -684,9 +697,9 @@ mod tests {
         assert_size!(ContentId, 12);
         assert_size!(Content, 216);
         assert_size!(Reason, 1);
-        assert_size!(ReasonElement, 16);
+        assert_size!(ReasonElement, 20);
         assert_size!(SessionId, 12);
-        assert_size!(Jingle, 104);
+        assert_size!(Jingle, 108);
     }
 
     #[cfg(target_pointer_width = "64")]
@@ -703,9 +716,9 @@ mod tests {
         #[cfg(feature = "stable")]
         assert_size!(Content, 440);
         assert_size!(Reason, 1);
-        assert_size!(ReasonElement, 32);
+        assert_size!(ReasonElement, 40);
         assert_size!(SessionId, 24);
-        assert_size!(Jingle, 208);
+        assert_size!(Jingle, 216);
     }
 
     #[test]
@@ -850,7 +863,7 @@ mod tests {
             Error::ParseError(string) => string,
             _ => panic!(),
         };
-        assert_eq!(message, "Reason contains a foreign element.");
+        assert_eq!(message, "Reason doesn’t contain a valid reason.");
 
         let elem: Element = "<jingle xmlns='urn:xmpp:jingle:1' action='session-initiate' sid='coucou'><reason><decline/></reason><reason/></jingle>".parse().unwrap();
         let error = Jingle::try_from(elem).unwrap_err();