HEP: conformance to C++20

[hkaiser]

This is a meta ticket collecting all issues related to conformance with C++20:

• Implement N4409 on top of HPX (Implement N4409 on top of HPX #1141)
• Adapt all parallel algorithms to Ranges TS (Adapt all parallel algorithms to Ranges TS #1668)
• Parallel algorithms should accept iterator/sentinel pairs (Parallel algorithms should accept iterator/sentinel pairs #3646)
• Add shift_left and shift_right algorithms (Add shift_left and shift_right algorithms #3706)
• Implement the C++20 Synchronization Library - barrier, latch, counting_semaphore, binary_semaphore, condition_variable, condition_variable_any, cv_status (Implement the C++20 Synchronization Library #3983, Adding C++20 counting semaphore API #4634)
• Implement C++20 Stop Token and Joining Threads (Implement C++20 Stop Token and Joining Threads #3994)
• Use C++20 std::source_location where available, adapt ours to conform (Use C++20 std::source_location where available, adapt ours to conform #3995, Use C++20 std::source_location, if available #5807)
• Adapt hpx::future to C++20 co_await (build error: HPX_WITH_DISTRIBUTED_RUNTIME=off is required for my configuration #4770)
• Adapt parallel algorithms to C++20 (Adapt parallel algorithms to C++20 #4822)
• Move algorithms to tag_fallback_dispatch (Move algorithms to tag_fallback_dispatch #5204)

Other issues in current code that need fixing for C++20:

• Add is_sentinel_for trait implementation and test (Add is_sentinel_for trait implementation and test #4745)
• Deprecate result_of and friends (Deprecate result_of and friends #4749)
• Add sized_sentinel_for concept and make distance dispatch according to that (Add sized_sentinel_for concept and make distance dispatch according to that #4768)
• Remove last use of std::result_of, removed in C++20 (Remove last use of std::result_of, removed in C++20 #4893)
• Move chrono functionality to hpx::chrono namespace (Move chrono functionality to hpx::chrono namespace #4917)
• Move public hpx::parallel::execution functionality to hpx::execution (Move public hpx::parallel::execution functionality to hpx::execution #4943)
• Moving is_execution_policy and friends into namespace hpx (Moving is_execution_policy and friends into namespace hpx #4969)
• Parallel sort fails to compile with C++20 (Parallel sort fails to compile with C++20 #4971)
• Removing use of get_temporary_buffer and return_temporary_buffer (Removing use of get_temporary_buffer and return_temporary_buffer #4972)
• hpx::parallel::fill fails compiling (hpx::parallel::fill fails compiling #5016)
• Lessen constraints on certain algorithm arguments (Lessen constraints on certain algorithm arguments #5027)

[hkaiser]

This can be closed now as all issues listed above have been resolved. Yay! Full conformance to C++20 achieved!

[gonidelis]

@hkaiser Thanks!!!