Switch to quick-xml

[GuillaumeGomez]

Fixes #1235.

I spent way too much time on this so I'll stop now. If now feels like taking over, here you go.

I think the whole parser should be rewritten so that we don't need the weird layout we currently have. However, be warned: quick-xml API is horrible which can lead to errors like this:

error[E0499]: cannot borrow `*buf` as mutable more than once at a time
  --> src/xmlparser.rs:75:42
   |
73 |     pub fn next_element<'a, 'b: 'a>(&mut self, buf: &'b mut Vec<u8>) -> Result<BytesStart<'a>, String> {
   |                         -- lifetime `'a` defined here
74 |         loop {
75 |             return match self.next_event(buf) {
   |                                          ^^^ `*buf` was mutably borrowed here in the previous iteration of the loop
76 |                 Ok(Event::Start(e)) => Ok(e),
   |                                        ----- returning this value requires that `*buf` is borrowed for `'a`

The macros I added could be useful.