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/openpgp.rs

@@ -67,8 +67,8 @@ mod tests {
         pubsub::{Item as PubSubItem, Publish},
         Item, NodeName,
     };
+    use core::str::FromStr;
     use minidom::Element;
-    use std::str::FromStr;
 
     #[test]
     fn pubsub_publish_pubkey_data() {