parsers: use Error type from xso

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.

parsers/src/delay.rs

@@ -32,10 +32,10 @@ impl PresencePayload for Delay {}
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::util::error::Error;
     use crate::Element;
     use jid::BareJid;
     use std::str::FromStr;
+    use xso::error::{Error, FromElementError};
 
     #[cfg(target_pointer_width = "32")]
     #[test]
@@ -71,7 +71,7 @@ mod tests {
             .unwrap();
         let error = Delay::try_from(elem.clone()).unwrap_err();
         let returned_elem = match error {
-            Error::TypeMismatch(_, _, elem) => elem,
+            FromElementError::Mismatch(elem) => elem,
             _ => panic!(),
         };
         assert_eq!(elem, returned_elem);
@@ -84,7 +84,7 @@ mod tests {
             .unwrap();
         let error = Delay::try_from(elem).unwrap_err();
         let message = match error {
-            Error::ParseError(string) => string,
+            FromElementError::Invalid(Error::Other(string)) => string,
             _ => panic!(),
         };
         assert_eq!(message, "Unknown child in delay element.");