SequentialCat for common case categorical construction

[OrestZborowski-SIG]

Often we have categoricals which are created by incrementing an int inside some numba function.
where:

  def SequentialCat(ids) -> rt.Cat:
        return rt.Categorical(ids, categories=np.arange(1, ids.max() + 1, dtype=np.int64))

This can work for any categorical that is in some finite [0, N] range that fits in memory, doesn’t need to be fully dense.