- 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`
