While the FromXml bound is necessary for the key implementation of the
XmlStream (the `impl Stream`), the AsXml bound is not as intrinsic: the
Sink implementation can take any AsXml implementation on each separate
send invocation.
By removing this bound, we can add extra logic *only* to the parsing,
without having to worry about effects on the sending side.
Concretely, we'll be able to create a wrapper type for XmppStreamElement
which allows us to capture errors during parsing, without having to
worry about what happens during serialisation.
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