syntax: use a better Show impl for Ident

Rather than just dumping the id in the interner, which is useless, actually
print the interned string. Adjust the lexer logging to use Show instead of
Poly.

src/libsyntax/parse/lexer/mod.rs

@@ -90,7 +90,7 @@ impl<'a> Reader for TtReader<'a> {
     }
     fn next_token(&mut self) -> TokenAndSpan {
         let r = tt_next_token(self);
-        debug!("TtReader: r={:?}", r);
+        debug!("TtReader: r={}", r);
         r
     }
     fn fatal(&self, m: &str) -> ! {