Add copy_backward and move_backward

[hkaiser]

This pull request also simplifies test::decorated_iterator

[hkaiser]

After thinking about this a bit more I realized that the only valid use case for copy_backward and move_backward is to handle overlapping source and destination ranges. However for parallel algorithms we have no guarantee in what sequence the elements are handled. Thus these two algorithms are useless (and there is a reason why these two are not part of N4071). Closing this without merging...

[StellarBot]

Test Failed.
Refer to this link for build results: http://hermione.cct.lsu.edu:5632/job/hpx_pr_debug_gcc_48_boost_1_55_0/69/

[StellarBot]

Test Passed.
Refer to this link for build results: http://hermione.cct.lsu.edu:5632/job/hpx_pr_debug_gcc_46_boost_1_55_0/79/

[StellarBot]

Test Failed.
Refer to this link for build results: http://hermione.cct.lsu.edu:5632/job/hpx_pr_debug_gcc_44_boost_1_55_0/99/

[StellarBot]

Test Failed.
Refer to this link for build results: http://hermione.cct.lsu.edu:5632/job/hpx_pr_debug_gcc_47_boost_1_55_0/68/