BACKPORT: simpler hashing of string-types

includes:

    ENH: simplify hashing overloads of string-types

    - this revises the changes made in 95cd8ee7 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`

    STYLE: use Hash<word> instead of string::hasher for runTimeSelectionTables