HPX V1.9.0 Namespace changes

The latest release includes amongst others changes in the namespaces so that HPX facilities correspond to the C++ Standard Library. The old namespaces are deprecated. Below is a comprehensive list of the namespace changes.

Table 186 Namespace changes in V1.9.0

Old namespace	New namespace
hpx::util::mem_fn	hpx::mem_fn
hpx::util::invoke	hpx::invoke
hpx::util::invoke_r	hpx::invoke_r
hpx::util::invoke_fused	hpx::invoke_fused
hpx::util::invoke_fused_r	hpx::invoke_fused_r
hpx::util::unlock_guard	hpx::unlock_guard
hpx::parallel::v1::reduce_by_key	hpx::experimental::reduce_by_key
hpx::parallel::v1::sort_by_key	hpx::experimental::sort_by_key
hpx::parallel::task_canceled_exception	hpx::experimental::task_canceled_exception
hpx::parallel::task_block	hpx::experimental::task_block
hpx::parallel::define_task_block	hpx::experimental::define_task_block |
hpx::parallel::define_task_block_restore_thread	hpx::experimental::define_task_block_restore_thread
hpx::execution::experimental::task_group	hpx::experimental::task_group