STYLE: simplify hashing to use struct instead of class

- more consistent with STL practices for function classes.

- string::hash function class now operates on std::string rather
  than Foam::string since we have now avoided inadvertent use of
  string conversion from int in more places.

src/OpenFOAM/primitives/strings/string/string.H

@@ -143,15 +143,14 @@ public:
         static const string null;
 
 
-    //- Hashing function class, shared by all the derived classes
-    class hash
+    //- Hashing function for string and derived string classes
+    struct hash
     {
-    public:
-        hash() = default;
-
-        //- Hash for string.
-        //  Uses Foam::string instead of std::string for automatic conversions.
-        inline unsigned operator()(const string& str, unsigned seed = 0) const;
+        inline unsigned operator()
+        (
+            const std::string& str,
+            unsigned seed = 0
+        ) const;
     };
 
 

src/OpenFOAM/primitives/strings/string/stringI.H

@@ -254,7 +254,7 @@ inline bool Foam::string::operator()(const std::string& text) const
 
 inline unsigned Foam::string::hash::operator()
 (
-    const string& str,
+    const std::string& str,
     unsigned seed
 ) const
 {