Design for namespaces

[mohe2015]

Also the namespace thing is probably not a final fix. Although I don't know XML really well I think we would need some mapping from namespace urls. Let me give an example (CalDAV):

<?xml version="1.0" encoding="utf-8"?>
<d:propfind xmlns:cal="urn:ietf:params:xml:ns:caldav" xmlns:cs="http://calendarserver.org/ns/" xmlns:d="DAV:" xmlns:nc="http://nextcloud.org/ns" xmlns:oc="http://owncloud.org/ns" xmlns:s="http://sabredav.org/ns">
  <d:prop xmlns:x1="http://apple.com/ns/ical/" xmlns:x2="http://nextcloud.com/ns">
    <d:current-user-principal />
  </d:prop>
</d:propfind>

should as far as I understand it should be equivalent to:

<?xml version="1.0" encoding="utf-8"?>
<dav:propfind xmlns:cal="urn:ietf:params:xml:ns:caldav" xmlns:cs="http://calendarserver.org/ns/" xmlns:dav="DAV:" xmlns:nc="http://nextcloud.org/ns" xmlns:oc="http://owncloud.org/ns" xmlns:s="http://sabredav.org/ns">
  <dav:prop xmlns:x1="http://apple.com/ns/ical/" xmlns:x2="http://nextcloud.com/ns">
    <dav:current-user-principal />
  </dav:prop>
</dav:propfind>

The way yaserde implements this is you specify a mapping of namespace-shorthands to namespace urls:

#[yaserde(
    namespace = "d: DAV:",
    namespace = "s: http://sabredav.org/ns",
    namespace = "cal: urn:ietf:params:xml:ns:caldav",
    namespace = "cs: http://calendarserver.org/ns/",
    namespace = "oc: http://owncloud.org/ns",
    namespace = "nc: http://nextcloud.org/ns"
)]

and then can use these shorthands. A potentially easier but maybe also annoying way would be to directly use the url maybe like:

#[serde(rename = "$unflatten=urn:ietf:params:xml:ns:caldav href")]

where we should choose delimiters as invalid characters of the namespace url. But then you would need to store all xmlns: attributes on a stack or so to know which shorthand this belongs to.

Originally posted by @mohe2015 in #306 (comment)

[Mingun]

I have a clearer vision how namespaces should be supported in the Deserializer. I'm working on that, so I close this as duplicate of #218