xso: use values instead of types for text codecs

This allows stateful or configurable codecs without having to express
all configuration in the type name itself. For example, we could have a
Base64 type with configurable Base64 engines without having to duplicate
the Base64 type itself.

(Note that the different engines in the Base64 crate are values, not
types.)

xso-proc/src/meta.rs

@@ -408,7 +408,7 @@ pub(crate) enum XmlFieldMeta {
     /// `#[xml(text)]`
     Text {
         /// The path to the optional codec type.
-        codec: Option<Type>,
+        codec: Option<Expr>,
     },
 
     /// `#[xml(child)`
@@ -497,7 +497,7 @@ impl XmlFieldMeta {
 
     /// Parse a `#[xml(text)]` meta.
     fn text_from_meta(meta: ParseNestedMeta<'_>) -> Result<Self> {
-        let mut codec: Option<Type> = None;
+        let mut codec: Option<Expr> = None;
         if meta.input.peek(Token![=]) {
             Ok(Self::Text {
                 codec: Some(meta.value()?.parse()?),