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/cert_management.rs

@@ -125,8 +125,8 @@ impl IqSetPayload for Revoke {}
 mod tests {
     use super::*;
     use crate::ns;
+    use core::str::FromStr;
     use minidom::Element;
-    use std::str::FromStr;
 
     #[cfg(target_pointer_width = "32")]
     #[test]