provide better variable names in write_term_to_chars/3 (#340)

src/prolog/machine/mod.rs

@@ -423,6 +423,16 @@ impl Machine {
                             )
         );
 
+
+        compile_user_module(&mut wam,
+                            Stream::from(CHARSIO),
+                            true,
+                            ListingSource::from_file_and_path(
+                                clause_name!("si"),
+                                lib_path.clone(),
+                            )
+        );
+
         if wam.compile_top_level().is_err() {
             panic!("Loading '$toplevel' module failed");
         }