Pare down the Synom trait

I would like to make it clearer that parsing a string is second-class
functionality compared to parsing tokens.
diff --git a/tests/common/mod.rs b/tests/common/mod.rs
index fcb8c61..5ffeb84 100644
--- a/tests/common/mod.rs
+++ b/tests/common/mod.rs
@@ -1,9 +1,12 @@
+#![allow(dead_code)]
+
 extern crate walkdir;
 
-use walkdir::DirEntry;
 use std::env;
 use std::u32;
 
+use self::walkdir::DirEntry;
+
 macro_rules! errorf {
     ($($tt:tt)*) => {
         {