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.
Since Rust 1.76, and some much older nightly, there have been
improvements to the niche computation, which leads to smaller types
which can encode the same amount of data, variants, and such.
This fixes the tests on this compiler version.
Version 0.21 replaced base64::decode() with an Engine trait and multiple
structs implementing it for various alphabets, various performance
profiles, etc. It is slightly longer to import but in the end does the
very same thing.
