xso: move helper iterators and builders into separate module

This declutters the main `xso` namespace. In addition, if (when) we
introduce more complex generic implementations, we might want to have
tests for these, and those can then live there, too, without making the
main `lib.rs` file gigantic (or moving the tests too far away from the
tested code).

xso/src/fromxml.rs

@@ -0,0 +1,60 @@
+//! # Generic builder type implementations
+//!
+//! This module contains [`FromEventsBuilder`] implementations for types from
+//! foreign libraries (such as the standard library).
+//!
+//! In order to not clutter the `xso` crate's main namespace, they are
+//! stashed away in a separate module.
+
+// Copyright (c) 2024 Jonas Schäfer <jonas@zombofant.net>
+//
+// 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 crate::error::{Error, FromEventsError};
+use crate::{FromEventsBuilder, FromXml};
+
+/// Helper struct to construct an `Option<T>` from XML events.
+pub struct OptionBuilder<T: FromEventsBuilder>(T);
+
+impl<T: FromEventsBuilder> FromEventsBuilder for OptionBuilder<T> {
+    type Output = Option<T::Output>;
+
+    fn feed(&mut self, ev: rxml::Event) -> Result<Option<Self::Output>, Error> {
+        self.0.feed(ev).map(|ok| ok.map(|value| Some(value)))
+    }
+}
+
+impl<T: FromXml> FromXml for Option<T> {
+    type Builder = OptionBuilder<T::Builder>;
+
+    fn from_events(
+        name: rxml::QName,
+        attrs: rxml::AttrMap,
+    ) -> Result<Self::Builder, FromEventsError> {
+        Ok(OptionBuilder(T::from_events(name, attrs)?))
+    }
+}
+
+/// Helper struct to construct an `Box<T>` from XML events.
+pub struct BoxBuilder<T: FromEventsBuilder>(Box<T>);
+
+impl<T: FromEventsBuilder> FromEventsBuilder for BoxBuilder<T> {
+    type Output = Box<T::Output>;
+
+    fn feed(&mut self, ev: rxml::Event) -> Result<Option<Self::Output>, Error> {
+        self.0.feed(ev).map(|ok| ok.map(|value| Box::new(value)))
+    }
+}
+
+impl<T: FromXml> FromXml for Box<T> {
+    type Builder = BoxBuilder<T::Builder>;
+
+    fn from_events(
+        name: rxml::QName,
+        attrs: rxml::AttrMap,
+    ) -> Result<Self::Builder, FromEventsError> {
+        Ok(BoxBuilder(Box::new(T::from_events(name, attrs)?)))
+    }
+}