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/Cargo.toml

@@ -23,3 +23,6 @@ gitlab = { repository = "xmpp-rs/xmpp-rs" }
 [dependencies]
 rxml = { version = "0.13.1", default-features = false, features = [ "std", "compact_str" ] }
 thiserror = "2.0"
+
+[dev-dependencies]
+rxml = { version = "0.13.1", default-features = false, features = [ "std", "compact_str", "macros" ] }