Analogous to the already existing `IntoXml` implementation helpers
based on an `Into<Element>` implementation on a type, this provides
the utilities for `AsXml`.
This is of course exceptionally inefficient, but it is only needed
transitionally until we have everything migrated to derive macros or
otherwise rewritten in terms of AsXml/FromXml.
This will soon replace the IntoXml trait. The idea here is that we
don't generally need to take ownership of values which are going to
be transformed into XML: most of the time, the XML text is created
by building a string from some more specific type, such as an
integer or an enum. Requiring to clone an entire structure for this
purpose is wasteful.
In other cases, we actually could reference data right from the structs
we are converting to XML. In those cases, assuming that an iterator
always generates owned data would be incorrect, too.
Hence, we introduce a new `Item` type which closely mirrors the
`rxml::Item` type, but where the constituents are `Cow`. In the upcoming
changes, we are going to work toward replacing all uses of `IntoXml`
with `AsXml`, as well as modifying the macros accordingly.
