minidom: breaking: Allow getting Element attributes by namespace

- Move Element.attributes to `AttrMap`, slowly using rxml's features and
  unrolling our own.
- Add Element::attr_ns that requires the attribute namespace.
  Element:attr defaults to rxml::Namespace::none() but the interface
  changes nonetheless for a &NcNameStr. Similar changes on
  `ElementBuilder` methods.
- Remove iterator structs for attributes, return a ref on the AttrMap
  directly as we don't need to keep attributes' internals hidden
  anymore.
- Enable rxml's `macros` feature within tests to access the `xml_ncname`
  macro.

Signed-off-by: pep <pep@bouah.net>

parsers/src/presence.rs

@@ -293,6 +293,7 @@ mod tests {
     use super::*;
     use jid::{BareJid, FullJid};
     use xso::error::FromElementError;
+    use xso::exports::rxml;
 
     #[cfg(target_pointer_width = "32")]
     #[test]
@@ -602,25 +603,31 @@ mod tests {
     fn presence_with_to() {
         let presence = Presence::new(Type::None);
         let elem: Element = presence.into();
-        assert_eq!(elem.attr("to"), None);
+        assert_eq!(elem.attr(rxml::xml_ncname!("to")), None);
 
         let presence = Presence::new(Type::None).with_to(Jid::new("localhost").unwrap());
         let elem: Element = presence.into();
-        assert_eq!(elem.attr("to"), Some("localhost"));
+        assert_eq!(elem.attr(rxml::xml_ncname!("to")), Some("localhost"));
 
         let presence = Presence::new(Type::None).with_to(BareJid::new("localhost").unwrap());
         let elem: Element = presence.into();
-        assert_eq!(elem.attr("to"), Some("localhost"));
+        assert_eq!(elem.attr(rxml::xml_ncname!("to")), Some("localhost"));
 
         let presence =
             Presence::new(Type::None).with_to(Jid::new("test@localhost/coucou").unwrap());
         let elem: Element = presence.into();
-        assert_eq!(elem.attr("to"), Some("test@localhost/coucou"));
+        assert_eq!(
+            elem.attr(rxml::xml_ncname!("to")),
+            Some("test@localhost/coucou")
+        );
 
         let presence =
             Presence::new(Type::None).with_to(FullJid::new("test@localhost/coucou").unwrap());
         let elem: Element = presence.into();
-        assert_eq!(elem.attr("to"), Some("test@localhost/coucou"));
+        assert_eq!(
+            elem.attr(rxml::xml_ncname!("to")),
+            Some("test@localhost/coucou")
+        );
     }
 
     #[test]