ENH: HashTable and HashSet improvements

- unfriend HashSet, HashTable IO operators

- global min(), max(), minMax() functions taking a labelHashSet and an
  optional limit. For example,

      labelHashSet set = ...;

      Info<< "min is " << min(set) << nl;
      Info<< "max (non-negative) " << max(set, 0) << nl;

- make HashTable iterator '->' dereferencing more consistent by also
  supporting non-pointer types as well.

- read HashTable values in-situ to avoid copying

applications/test/HashSet/Test-hashSet.C

@@ -32,6 +32,7 @@ Description
 #include "HashOps.H"
 #include "HashSet.H"
 #include "Map.H"
+#include "MinMax.H"
 #include "labelPairHashes.H"
 #include "FlatOutput.H"
 
@@ -184,6 +185,9 @@ int main(int argc, char *argv[])
         1, 11, 42
     };
 
+    Info<<"Set with min/max:" << minMax(setB)
+        << " min:" << min(setB) << " max:" << max(setB) << nl;
+
     setB = FixedList<label, 4>({1, 2, 3, 4});
     setB = {1, 2, 4};
     setB = List<label>({1, 2, 4});