parsers: use Error type from xso

This is a large change and as such, it needs good motivation. Let me
remind you of the ultimate goal: we want a derive macro which allows us
to FromXml/IntoXml, and that derive macro should be usable from
`xmpp_parsers` and other crates.

For that, any code generated by the derive macro mustn't depend on any
code in the `xmpp_parsers` crate, because you cannot name the crate you
are in portably (`xmpp_parsers::..` wouldn't resolve within
`xmpp_parsers`, and `crate::..` would point at other crates if the macro
was used in other crates).

We also want to interoperate with code already implementing
`TryFrom<Element>` and `Into<Element>` on structs. This ultimately
requires that we have an error type which is shared by the two
implementations and that error type must be declared in the `xso` crate
to be usable by the macros.

Thus, we port the error type over to use the type declared in `xso`.

This changes the structure of the error type greatly; I do not think
that `xso` should have to know about all the different types we are
parsing there and they don't deserve special treatment. Wrapping them in
a `Box<dyn ..>` seems more appropriate.

xso/src/minidom_compat.rs

@@ -55,7 +55,7 @@ enum IntoEventsInner {
 // NOTE to developers: The limitations are not fully trivial to overcome:
 // the attributes use a BTreeMap internally, which does not offer a `drain`
 // iterator.
-fn make_start_ev_parts(el: &Element) -> Result<(rxml::QName, AttrMap), Error> {
+pub fn make_start_ev_parts(el: &Element) -> Result<(rxml::QName, AttrMap), Error> {
     let name = NcName::try_from(el.name())?;
     let namespace = Namespace::from(el.ns());