feat(users/Profpatsch/netencode): encode() impl for T and U

Also change the toplevel `encode()` to take a `&U` instead of an owned
`U`.

Change-Id: I8e51540cc531e70ae1c94e3676f4dd88da7a924d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2491
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

users/Profpatsch/read-http.rs

@@ -171,7 +171,7 @@ fn write_dict<'buf, 'a>(mut http: HashMap<&str, U<'a>>, headers: &'a HashMap<Str
     };
     netencode::encode(
         &mut std::io::stdout(),
-        U::Record(http)
+        &U::Record(http)
     )?;
     Ok(())
 }