xso-proc: add support for text codecs

Text codecs allow to customize the conversion of data from/to XML,
in particular in two scenarios:

1. When the type for which the behaviour is to be defined comes from a
   foreign crate, preventing the implementation of
   FromXmlText/IntoXmlText.

2. When there is not one obvious, or more than one sensible, way to
   convert a value to XML text and back.

xso/src/lib.rs

@@ -24,7 +24,7 @@ pub mod error;
 #[cfg(feature = "minidom")]
 #[cfg_attr(docsrs, doc(cfg(feature = "minidom")))]
 pub mod minidom_compat;
-mod text;
+pub mod text;
 
 #[doc(hidden)]
 pub mod exports {
@@ -35,6 +35,9 @@ pub mod exports {
 
 use std::borrow::Cow;
 
+#[doc(inline)]
+pub use text::TextCodec;
+
 #[doc = include_str!("from_xml_doc.md")]
 #[doc(inline)]
 #[cfg(feature = "macros")]