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.
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