Fix S/R versions of the parallel algorithms

[zhekemist]

PR #6494 adds unit test cases and fixes for the S/R versions of a subset of HPX's parallel algorithms. This issue serves to track the remaining algorithms that have not yet been adapted to work with S/R and/or do not yet have unit test cases for their S/R versions:

• copy_if
• exclusive_scan
• for_loop (see PR Fix the S/R versions of the for loop algorithms #6529)
• for_loop_n (see PR Fix the S/R versions of the for loop algorithms #6529)
• for_loop_strided (see PR Fix the S/R versions of the for loop algorithms #6529)
• for_loop_n_strided (see PR Fix the S/R versions of the for loop algorithms #6529)
• inclusive_scan
• make_heap
• merge
• inplace_merge
• nth_element
• partial_sort
• partial_sort_copy
• stable_partition
• partition
• partition_copy
• reduce_by_key
• remove_copy
• remove_copy_if
• search_n
• set_difference
• set_intersection
• set_symmetric_difference
• set_union
• shift_left
• shift_right
• sort
• sort_by_key
• stable_sort
• transform_exclusive_scan
• transform_inclusive_scan
• uninitialized_copy
• uninitialized_copy_n
• uninitialized_default_construct
• uninitialized_default_construct_n
• uninitialized_fill
• unititialized_fill_n
• uninitialized_move
• uninitialized_move_n
• uninitialized_relocate
• uninitialized_relocate_backward
• uninitialized_relocate_n
• uninitialized_value_construct
• uninitialized_value_construct_n
• unique_copy

Note: During the work on PR #6494, S/R unit tests were added for some algorithms without also fixing their S/R versions. To avoid cluttering the tests, these temporarily non-functional unit test cases were removed. However, if they are needed for future work, you they can be restored by reverting commit a20f1b4. See this spreadsheet from the PR for a list of algorithms for which unit test cases were originally added.