Adding executor parallel invoke CPOs

[hkaiser]

This adds new CPOs: sync_invoke and async_invoke. It also adds an implementation for the fork_join_executor and block_fork_join_executor as well as fallback implementations for all executors not supporting these. The APIs are:

namespace hpx::parallel::execution {

    template <typename Executor, typename F, typename... Fs,
        HPX_CONCEPT_REQUIRES_(
            std::is_invocable_v<F> && (std::is_invocable_v<Fs> && ...)
        )>
    void sync_invoke(Executor&& exec, F&& f, Fs&&... fs);

    template <typename Executor, typename F, typename... Fs,
        HPX_CONCEPT_REQUIRES_(
            std::is_invocable_v<F> && (std::is_invocable_v<Fs> && ...)
        )>
    decltype(auto) async_invoke(Executor&& exec, F&& f, Fs&&... fs);
}

For executors supporting the CPOs this will concurrently launch the given functions and will either wait for them to complete or return a completion token (e.g., a hpx::future) representing the completion of the function invocations.

[hkaiser]

bors merge

[bors]

This PR was included in a batch that successfully built, but then failed to merge into master (it was a non-fast-forward update). It will be automatically retried.

[bors]

Build succeeded!

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• Bors