xmpp-parsers: Don’t use anything from std

The remaining two were core::str::FromStr and
alloc::collections::BTreeMap.

Note that this doesn’t allow us to be no_std yet.

skip-changelog: This isn’t in any way user-visible, except that some
structs are now slightly smaller.

parsers/src/idle.rs

@@ -24,8 +24,8 @@ impl PresencePayload for Idle {}
 #[cfg(test)]
 mod tests {
     use super::*;
+    use core::str::FromStr;
     use minidom::Element;
-    use std::str::FromStr;
     use xso::error::{Error, FromElementError};
 
     #[test]