This introduces a breaking change by moving from a bool to an
Option<NoCertManagement>, which will be reverted eventually once we add
support for #[xml(flag)] types of children.

This will allow other kinds of enumerations in the future more
easily.

These can be a simple String for now.

This allows servers to communicate their maximum stanza limit, as well
as idle time limit.

Jid("foo@bar/baz") instead of Jid { normalized: "foo@bar/baz", at:
Some(3), slash: Some(7) }, which I find much more readable.

Same for BareJid and FullJid.

This is different from the bind query or the bind response, despite all
three having the same element namespace and name…  Annoying!

This is nicer than sleeping forever with no debug info whatsoever.

This had been forgotten when I converted it to xso.

Especially exhaustive enums are super useful for this kind of pattern.

This allows such default type wrappers to be used in other xso elements.

This decreases indentation levels on the various implemenations, it
groups the implementations together physically, and (spoiler alert!)
we'll actually need the dyn Field trait object-ness (much) later on.

This is in prepraration of more refactoring to come, in order to make
the individual field implementations easier to grok and look at.

This should make everything a little easier to read, because it reduces
the level of indentation involved.

We can *finally* do it!