Make TryFrom<Element> chainable

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.

parsers/src/blocking.rs

@@ -37,7 +37,7 @@ macro_rules! generate_blocking_element {
                 check_no_attributes!(elem, $name);
                 let mut items = vec!();
                 for child in elem.children() {
-                    check_self!(child, "item", BLOCKING);
+                    check_child!(child, "item", BLOCKING);
                     check_no_unknown_attributes!(child, "item", ["jid"]);
                     check_no_children!(child, "item");
                     items.push(get_attr!(child, "jid", Required));