Implement `on_result` execution adaptor

[rrahn]

Can be put probably under core/algorithm/execution/

Required by #344

From the current design this could be the simplest implmentation to allow asynchron execution for the algorithms. One could outsource this into a separate data structure to reuse it for alignment and search.

Note this is based on changes made in #1854:

auto algo(range r, config)
{
    ...
    auto algorithm = configure(config);
    ...
    constexpr bool requires_one_way_execution = config::exists<algo_config::on_result>();
    if constexpr (!requires_one_way_execution)
    {  // Current behavior
        return range{algorithm_executor_blocking{...}};
    }
    else
    { // New behavior
       auto executor = select_execution_handler();
       executor.bulk_execute(algorithm, r, user_callback); // implicit wait :)
       //for (auto && elem : r)
       //    executor.execute(algorithm, elem, user_callback);

       // exector.wait();
    }
}

[rrahn]

Will be planned for the alignment changes