Adapt execution policy name changes from C++17

[hkaiser]

While the Parallelsim TS has been now adopted tobe part of C++17, there have been a couple of name changes which require corresponding changes in HPX:

• par_vec has been renamed to par_unseq (see Adapt execution policy name changes from C++17 #2436)
• execution policies now live in namespace std::execution (see Adapt execution policy name changes from C++17 #2436)
• execution policies now don't have 'execution' in their name anymore (i.e. the former std::sequential_execution_policy is now named std::execution::sequential_policy) (see Adapt execution policy name changes from C++17 #2436)
• the header <execution_policy> is now named <execution> (see Adapt execution policy name changes from C++17 #2436)
• inner_product was renamed to transform_reduce (see Adapt algorithm parameter sequence changes from C++17 #2437)
• various parameter sequence changes have been adopted (for transform_reduce etc.) (see Adapt algorithm parameter sequence changes from C++17 #2437)

Other changes (we need to decide whether we will adopt those):

- [ ] any exception thrown from any of the algorithms now calls std::terminate
- [ ] the algorithm adjacent_difference has been removed

[biddisco]

"any exception thrown from any of the algorithms now calls std::terminate"

Why on earth was this decided. I agree that its hard to recover from an error, but calling terminate sounds very drastic.

[hkaiser]

Why on earth was this decided. I agree that its hard to recover from an error, but calling terminate sounds very drastic.

I agree completely and I don't think we should follow down that path. The way the wording is done now is that the exception behavior is defined by the execution policy and all standardized policies (std::execution::seq, std::execution::par, and std::execution::par_unseq) will cause for the algorithms to call std::terminate in case of any exceptions. This leaves us free to do whatever seems to be appropriate.

The rationale was that there was not sufficient time for C++17 to come up with a (proper) solution. Any other half-solution would have caused breaking changes down the road. Since par_unseq is not implementable using exceptions (and the spec always mandated calling std::terminate in this case), the majority opinion was to unify the behavior. The alternative would have been to completely rip out the parallel algorithms.