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/sasl.rs

@@ -7,7 +7,7 @@
 use xso::{text::Base64, AsXml, FromXml};
 
 use crate::ns;
-use std::collections::BTreeMap;
+use alloc::collections::BTreeMap;
 
 generate_attribute!(
     /// The list of available SASL mechanisms.