ENH: simplify hashing overloads of string-types

- this revises the changes made in 95cd8ee75c to replace the
  SFINAE-type of handling of string hashes with direct definitions.

  This places a bit more burden on the developer if creating hashable
  classes derived from std::string or variants of Foam::string, but
  improves reliability when linking.

STYLE: drop template key defaulting from HashSet

- this was never used and `HashSet<>` is much less transparent
  than writing `HashSet<word>` or `wordHashSet`

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

@@ -62,11 +62,14 @@ namespace Foam
 {
 
 // Forward Declarations
+class string;
 class word;
 class wordRe;
 class Istream;
 class Ostream;
 
+template<class T> struct Hash;
+
 /*---------------------------------------------------------------------------*\
                            Class string Declaration
 \*---------------------------------------------------------------------------*/
@@ -150,7 +153,8 @@ public:
             }
         };
 
-        //- Hashing functor for string and derived string classes
+        //- Deprecated hashing functor - use hasher
+        //  \deprecated(2021-04) - use hasher
         struct hash : string::hasher {};
 
 
@@ -333,6 +337,17 @@ public:
 };
 
 
+// * * * * * * * * * * * * * * * Global Functions  * * * * * * * * * * * * * //
+
+//- Hashing for Foam::string
+template<> struct Hash<string> : string::hasher {};
+
+//- Hashing for std:::string
+template<> struct Hash<std::string> : string::hasher {};
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
 // IOstream Operators
 
 //- Read operator