xso: Add std feature ; default-features compiles in no-std

xso/src/minidom_compat.rs

@@ -5,9 +5,13 @@
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+use alloc::{
+    borrow::{Cow, ToOwned},
+    boxed::Box,
+    vec::IntoIter,
+};
 use core::marker::PhantomData;
-use std::borrow::Cow;
-use std::vec::IntoIter;
 
 use minidom::{Element, Node};