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>

minidom/CHANGELOG.md

@@ -2,6 +2,12 @@ Version NEXT:
   * Breaking:
     * Removed 'std' feature for now because it makes build fail on
       no-default-features.
+    * Add `Element::attr_ns` that requires the attribute namespace in addition
+      to `Element::attr` (which defaults to `rxml::Namespace::none()`. Similar
+      changes on `ElementBuilder` methods.
+      `Element` now uses `AttrMap` to store attributes, and this also implies
+      some more changes in the Element interface such as iterators on
+      attributes.
   * Changes
     * Use thiserror for the error type. (!616)