Add result_type member type to dpp::task

[BowDown097]

Pretty useful when working with templates (for example, this code I wrote, where I had to reproduce this with a false_type/true_type struct). Similar conveniences are part of quite a few STL template types too, namely std::function which I'd say is pretty similar.

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[Mishura4]

Thank you for the contribution, that's a good idea for sure! Any chance you could add it to dpp::coroutine and dpp::async as well?