Namespace handling

[singpolyma]

We may not want to use our own Stanza/XML element implementation at all (since I expect it to be more performant to pass through to the underlying one) but assuming we do we'll need to be careful about it.

For example https://github.com/snikket-im/snikket-sdk/blob/main/xmpp/Stanza.hx#L126 is already buggy WRT namespace handling, assuming that if a namespace is specified it must be present as an xmlns attribute on the child (and not inherited from parent, grandparent, set by way of namespace prefix, etc).