Mark OLESEN authored Apr 12, 2021 and Andrew Heather committed Apr 19, 2021

- additional dummy template parameter to assist with supporting
  derived classes. Currently just used for string types, but can be
  extended.

- provide hash specialization for various integer types.
  Removes the need for any forwarding.

- change default hasher for HashSet/HashTable from 'string::hash'
  to `Hash<Key>`. This avoids questionable hashing calls and/or
  avoids compiler resolution problems.

  For example,
  HashSet<label>::hasher and labelHashSet::hasher now both properly
  map to Hash<label> whereas previously HashSet<label> would have
  persistently mapped to string::hash, which was incorrect.

- standardize internal hashing functors.

  Functor name is 'hasher', as per STL set/map and the OpenFOAM
  HashSet/HashTable definitions.

  Older code had a local templated name, which added unnecessary
  clutter and the template parameter was always defaulted.
  For example,

      Old:  `FixedList<label, 3>::Hash<>()`
     ...