regex: print out final regex in trace mode

This is useful for debugging to see what regex is actually being run.
We put this as a trace since the regex can be quite gnarly. (It is not
pretty printed.)

grep-regex/src/word.rs

@@ -55,6 +55,11 @@ impl WordMatcher {
         }
         Ok(WordMatcher { regex, names, locs })
     }
+
+    /// Return the underlying regex used by this matcher.
+    pub fn regex(&self) -> &Regex {
+        &self.regex
+    }
 }
 
 impl Matcher for WordMatcher {