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/expressions/exprString/exprString.H

@@ -5,8 +5,8 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Original code Copyright (C) 2012-2018 Bernhard Gschaider
-    Copyright (C) 2019 OpenCFD Ltd.
+    Copyright (C) 2012-2018 Bernhard Gschaider
+    Copyright (C) 2019-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -65,7 +65,7 @@ public:
 
     // Constructors
 
-        //- Construct null
+        //- Default construct
         exprString() = default;
 
         //- Copy construct
@@ -189,6 +189,16 @@ public:
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 } // End namespace expressions
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+//- Hashing for exprString is the same as string
+template<> struct Hash<expressions::exprString> : string::hasher {};
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
 } // End namespace Foam
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //