Support non-UTF-8 encodings

[jonas-schievink]

We're currently using xml-rs to parse XML, which only supports UTF-8 (netvl/xml-rs#10).

To do this, either xml-rs needs to get support for other encodings or we have to switch to quick-xml which does support non-UTF-8 documents (but needs some work - it needs to be better tested and doesn't support encodings that aren't ASCII-compatible).

Note that the XML-spec mandates UTF-8 and UTF-16 support, so xml-rs is technically non-compliant (as is this crate).