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/pubsub/owner.rs

@@ -192,8 +192,8 @@ impl From<PubSubOwner> for Element {
 mod tests {
     use super::*;
     use crate::data_forms::{DataFormType, Field, FieldType};
+    use core::str::FromStr;
     use jid::BareJid;
-    use std::str::FromStr;
 
     #[test]
     fn affiliations() {