Fix required literal handling and add debug prints.

In particular, if we had an inner literal and were doing a case insensitive
search, then the literals are dropped because we previously only allowed
a single inner literal to have an effect. Now we allow alternations of
inner literals, but still don't quite take full advantage.

grep/src/lib.rs

@@ -4,6 +4,8 @@
 A fast line oriented regex searcher.
 */
 
+#[macro_use]
+extern crate log;
 extern crate memchr;
 extern crate regex;
 extern crate regex_syntax as syntax;