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.)

parsers/src/component.rs

@@ -20,7 +20,7 @@ pub struct Handshake {
     ///
     /// If None, it is the successful reply from the server, the stream is now
     /// fully established and both sides can now exchange stanzas.
-    #[xml(text(codec = FixedHex<20>))]
+    #[xml(text(codec = FixedHex::<20>))]
     pub data: Option<[u8; 20]>,
 }