ENH: add missing Hash function for List/UList (issue #966)

- there were previously no hashing mechanisms for lists so they
  would fall back to the definition for primitives and hash the
  memory location of the allocated List object.

- provide a UList::Hash<> sub-class for inheritance, and also a global
  specialization for UList<T>, List<T> such that the hash value for
  List<List<T>> cascades properly.

- provide similar function in triFace to ensure that it remains
  similar in behaviour to face.

- added SymmHash to Pair, for use when order is unimportant.

STYLE: use string::hash() more consistently

- no particular reason to use Hash<word>() which forwards to
  string::hash() anyhow

applications/test/List/Test-List.C

@@ -598,6 +598,20 @@ int main(int argc, char *argv[])
         << "-wordList:   " << flatOutput(wLst)  << nl
         << "-stringList: " << flatOutput(sLst)  << endl;
 
+    // Hash values
+    {
+        labelList list1(identity(5));
+        labelList list2(identity(5));
+
+        Info<<"hash of " << flatOutput(list1)
+            << " = " << Hash<labelList>()(list1) << " or "
+            << labelList::Hash<>()(list1) << nl;
+
+        Info<<"hash of " << flatOutput(list2) << " = "
+            << Hash<labelList>()(list2) << " or "
+            << labelList::Hash<>()(list2) << nl;
+    }
+
     return 0;
 }