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.
This allows constructs like:
```rust
let residual = match Iq::try_from(stanza) {
Ok(iq) => return handle_iq(..),
Err(Error::TypeMismatch(_, _, v)) => v,
Err(other) => return handle_parse_error(..),
};
let residual = match Message::try_from(stanza) {
..
};
let residual = ..
log::warn!("unhandled object: {:?}", residual);
```
The interesting part of this is that this could be used in a loop over a
Vec<Box<dyn FnMut(Element) -> ControlFlow<SomeResult, Element>>, i.e. in
a parsing loop for a generic XML/XMPP stream.
The advantage is that the stanza.is() check runs only once (in
check_self!) and doesn't need to be duplicated outside, and it reduces
the use of magic strings.
