xso-proc: Add the default flag to the element meta

This allows the payload to be absent, and requires the field type to be
Option<minidom::Element>.

xso/ChangeLog

@@ -22,7 +22,8 @@ Version NEXT:
         structs.
       - Support for collecting all unknown children in a single field as
         collection of `minidom::Element`, or one unknown child as a
-        `minidom::Element`.
+        `minidom::Element`, or zero or one unknown children as an
+        `Option<minidom::Element>`.
       - Support for "transparent" structs (newtype-like patterns for XSO).
       - FromXmlText and AsXmlText are now implemented for jid::NodePart,
         jid::DomainPart, and jid::ResourcePart (!485)

xso/src/from_xml_doc.md

@@ -433,6 +433,7 @@ The following keys can be used inside the `#[xml(extract(..))]` meta:
 
 | Key | Value type | Description |
 | --- | --- | --- |
+| `default` | flag | If present, an absent child will substitute the default value instead of raising an error. |
 | `n` | `1` or `..` | If `1`, a single element is parsed. If `..`, a collection is parsed. Defaults to `1`. |
 
 When parsing a single child element (i.e. `n = 1` or no `n` value set at all),
@@ -446,6 +447,16 @@ addition, the field's type must implement
 field's reference type must implement
 `IntoIterator<Item = &'_ minidom::Element>` to derive `AsXml`.
 
+If `default` is specified and the child is absent in the source, the value
+is generated using [`core::default::Default`]. `default` has no influence on
+`AsXml`. Combining `default` and `n` where `n` is not set to `1` is not
+supported and will cause a compile-time error.
+
+Using `default` with a type other than `Option<T>` will cause the
+serialisation to mismatch the deserialisation (i.e. the struct is then not
+roundtrip-safe), because the deserialisation does not compare the value
+against `default` (but has special provisions to work with `Option<T>`).
+
 Fields with the `element` meta are deserialised with the lowest priority.
 While other fields are processed in the order they are declared, `element`
 fields may capture arbitrary child elements, so they are considered as the