ENH: improve hashing overloads of string-types and HashTable/HashSet

- additional dummy template parameter to assist with supporting
  derived classes. Currently just used for string types, but can be
  extended.

- provide hash specialization for various integer types.
  Removes the need for any forwarding.

- change default hasher for HashSet/HashTable from 'string::hash'
  to `Hash<Key>`. This avoids questionable hashing calls and/or
  avoids compiler resolution problems.

  For example,
  HashSet<label>::hasher and labelHashSet::hasher now both properly
  map to Hash<label> whereas previously HashSet<label> would have
  persistently mapped to string::hash, which was incorrect.

- standardize internal hashing functors.

  Functor name is 'hasher', as per STL set/map and the OpenFOAM
  HashSet/HashTable definitions.

  Older code had a local templated name, which added unnecessary
  clutter and the template parameter was always defaulted.
  For example,

      Old:  `FixedList<label, 3>::Hash<>()`
      New:  `FixedList<label, 3>::hasher()`
      Unchanged:  `labelHashSet::hasher()`

  Existing `Hash<>` functor namings are still supported,
  but deprecated.

- define hasher and Hash specialization for bitSet and PackedList

- add symmetric hasher for 'face'.
  Starts with lowest vertex value and walks in the direction
  of the next lowest value. This ensures that the hash code is
  independent of face orientation and face rotation.

NB:
  - some of keys for multiphase handling (eg, phasePairKey)
    still use yet another function naming: `hash` and `symmHash`.
    This will be targeted for alignment in the future.

applications/solvers/multiphase/compressibleMultiphaseInterFoam/multiphaseMixtureThermo/multiphaseMixtureThermo.H

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2013-2017 OpenFOAM Foundation
-    Copyright (C) 2019 OpenCFD Ltd.
+    Copyright (C) 2019-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -59,24 +59,23 @@ class multiphaseMixtureThermo
 {
 public:
 
+    //- Symmetric pair of interface names
     class interfacePair
     :
         public Pair<word>
     {
     public:
 
-        struct hash
-        {
-            label operator()(const interfacePair& key) const
-            {
-                return word::hash()(key.first()) + word::hash()(key.second());
-            }
-        };
+        // Always use symmetric hashing
+        using hasher = Pair<word>::symmHasher;
+
+        // Always use symmetric hashing (alias)
+        using hash = Pair<word>::symmHasher;
 
 
         // Constructors
 
-            interfacePair() {} // = default
+            interfacePair() = default;
 
             interfacePair(const word& alpha1Name, const word& alpha2Name)
             :
@@ -97,11 +96,7 @@ public:
                 const interfacePair& b
             )
             {
-                return
-                (
-                    ((a.first() == b.first()) && (a.second() == b.second()))
-                 || ((a.first() == b.second()) && (a.second() == b.first()))
-                );
+                return (0 != Pair<word>::compare(a, b));
             }
 
             friend bool operator!=
@@ -117,7 +112,7 @@ public:
 
 private:
 
-    // Private data
+    // Private Data
 
         //- Dictionary of phases
         PtrDictionary<phaseModel> phases_;
@@ -140,7 +135,7 @@ private:
         const dimensionedScalar deltaN_;
 
 
-    // Private member functions
+    // Private Member Functions
 
         void calcAlphas();
 

applications/solvers/multiphase/multiphaseInterFoam/multiphaseMixture/multiphaseMixture.H

@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
+    Copyright (C) 2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -67,24 +68,23 @@ class multiphaseMixture
 {
 public:
 
+    //- Symmetric pair of interface names
     class interfacePair
     :
         public Pair<word>
     {
     public:
 
-        struct hash
-        {
-            label operator()(const interfacePair& key) const
-            {
-                return word::hash()(key.first()) + word::hash()(key.second());
-            }
-        };
+        // Always use symmetric hashing
+        using hasher = Pair<word>::symmHasher;
+
+        // Always use symmetric hashing (alias)
+        using hash = Pair<word>::symmHasher;
 
 
         // Constructors
 
-            interfacePair() {} // = default
+            interfacePair() = default;
 
             interfacePair(const word& alpha1Name, const word& alpha2Name)
             :
@@ -105,11 +105,7 @@ public:
                 const interfacePair& b
             )
             {
-                return
-                (
-                    ((a.first() == b.first()) && (a.second() == b.second()))
-                 || ((a.first() == b.second()) && (a.second() == b.first()))
-                );
+                return (0 != Pair<word>::compare(a, b));
             }
 
             friend bool operator!=