This is a large change and as such, it needs good motivation. Let me
remind you of the ultimate goal: we want a derive macro which allows us
to FromXml/IntoXml, and that derive macro should be usable from
`xmpp_parsers` and other crates.
For that, any code generated by the derive macro mustn't depend on any
code in the `xmpp_parsers` crate, because you cannot name the crate you
are in portably (`xmpp_parsers::..` wouldn't resolve within
`xmpp_parsers`, and `crate::..` would point at other crates if the macro
was used in other crates).
We also want to interoperate with code already implementing
`TryFrom<Element>` and `Into<Element>` on structs. This ultimately
requires that we have an error type which is shared by the two
implementations and that error type must be declared in the `xso` crate
to be usable by the macros.
Thus, we port the error type over to use the type declared in `xso`.
This changes the structure of the error type greatly; I do not think
that `xso` should have to know about all the different types we are
parsing there and they don't deserve special treatment. Wrapping them in
a `Box<dyn ..>` seems more appropriate.
[gone](https://datatracker.ietf.org/doc/html/rfc6120#section-8.3.3.5) and [redirect](https://datatracker.ietf.org/doc/html/rfc6120#section-8.3.3.14) errors may include an alternative address.
> gone
>
> The recipient or server can no longer be contacted at this address,
> typically on a permanent basis (as opposed to the <redirect/> error
> condition, which is used for temporary addressing failures); the
> associated error type SHOULD be "cancel" and the error stanza SHOULD
> include a new address (if available) as the XML character data of the
> <gone/> element (which MUST be a Uniform Resource Identifier [URI] or
> Internationalized Resource Identifier [IRI] at which the entity can
> be contacted, typically an XMPP IRI as specified in [XMPP-URI]).
—
> redirect
>
> The recipient or server is redirecting requests for this information
> to another entity, typically in a temporary fashion (as opposed to
> the <gone/> error condition, which is used for permanent addressing
> failures); the associated error type SHOULD be "modify" and the error
> stanza SHOULD contain the alternate address in the XML character data
> of the <redirect/> element (which MUST be a URI or IRI with which the
> sender can communicate, typically an XMPP IRI as specified in
> [XMPP-URI](https://datatracker.ietf.org/doc/html/rfc6120#ref-XMPP-URI)).
This moves InnerJid into Jid and reformulates BareJid and FullJid in
terms of Jid.
Doing this has the key advantage that FullJid and BareJid can deref to
and borrow as Jid. This, in turn, has the advantage that they can be
used much more flexibly in HashMaps. However, this is (as we say in
Germany) future music; this commit only does the internal reworking.
Oh and also, it saves 20% memory on Jid objects.
Fixes#122 more thoroughly, or rather the original intent behind it.
