BUG: incorrect HashTable / HashSet instances

- Eg instead of using labelHashSet, used HashSet<label> which uses
  the string::hash for hashing. Other places inadvertently using the
  string::hash instead of Hash<label> for hashing.

STYLE: use Map<..> instead of HashTable<.., label, Hash<label>>

- reduces clutter

applications/test/HashSet/Test-hashSet.C

@@ -56,7 +56,7 @@ int main(int argc, char *argv[])
 
     setA = { "kjhk", "kjhk2", "abced" };
 
-    HashTable<label, word> tableA
+    HashTable<label> tableA
     {
         { "value1", 1 },
         { "value2", 2 },