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/fileFormats/ensight/name/ensightFileName.H

@@ -106,6 +106,15 @@ public:
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 } // End namespace ensight
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+//- Hashing for FileName is the same as string
+template<> struct Hash<ensight::FileName> : string::hasher {};
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
 } // End namespace Foam
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

src/fileFormats/ensight/name/ensightVarName.H

@@ -104,6 +104,16 @@ public:
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 } // End namespace ensight
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+//- Hashing for VarName is the same as string
+template<> struct Hash<ensight::VarName> : string::hasher {};
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
 } // End namespace Foam
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //