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/extdisco.rs

@@ -193,7 +193,7 @@ mod tests {
         let query = ServicesQuery { type_: None };
         let elem = Element::from(query);
         assert!(elem.is("services", ns::EXT_DISCO));
-        assert_eq!(elem.attrs().next(), None);
+        assert_eq!(elem.attrs().into_iter().next(), None);
         assert_eq!(elem.nodes().next(), None);
     }