Add a primitive debug tracing facility.

When the CRETONNE_DBG environment variable is set, send debug messages
to a file named cretonne.dbg.*.

The trace facility is only enabled when debug assertions are on.

src/cton-util.rs

@@ -1,4 +1,4 @@
-
+#[macro_use(dbg)]
 extern crate cretonne;
 extern crate cton_reader;
 extern crate docopt;