Function even_share

include/exec/static_thread_pool.hpp:90–105

    // ```
    template <class Shape>
    constexpr auto even_share(Shape n, std::size_t rank, std::size_t size) noexcept  //
      -> std::pair<Shape, Shape>
    {
      STDEXEC_ASSERT(n >= 0);
      using ushape_t            = std::make_unsigned_t<Shape>;
      auto const avg_per_thread = static_cast<ushape_t>(n) / size;
      auto const n_big_share    = avg_per_thread + 1;
      auto const big_shares     = static_cast<ushape_t>(n) % size;
      auto const is_big_share   = rank < big_shares;
      auto const begin          = is_big_share
                                  ? n_big_share * rank
                                  : n_big_share * big_shares + (rank - big_shares) * avg_per_thread;
      auto const end            = begin + (is_big_share ? n_big_share : avg_per_thread);

      return std::make_pair(static_cast<Shape>(begin), static_cast<Shape>(end));
    }

    namespace schedule_all_
    {