This adds shims which provide FromXml and IntoXml implementations to
*all* macro-generated types in `xmpp_parsers`. Mind that this does not
cover all types in `xmpp_parsers`, but a good share of them.
This is another first step toward real, fully streamed parsing.
This library provides the traits to parse structs from XML and
serialise them into XML without having to buffer the document object
model in memory.
The only implementations it provides are for minidom, basically
providing a lower-level interface to `minidom::Element::from_reader` and
`minidom::Element::to_writer`.
This is the first stepping stone into a world where `xmpp_parsers` can
parse the structs directly from XML.
