Change namespaces of Public API

[dimitraka]

This is a list of the names that need to change from sub-namespaces to the hpx namespace. So far the list consists of the following:

• [x] hpx::traits::is_bind_expression
• [x] hpx::traits::is_placeholder
• [x] hpx::experimental::bind_back
• [x] hpx::util::mem_fn (see #6067)
• [x] hpx::util::invoke, hpx::util::invoke_r (see #6048)
• [x] hpx::util::invoke_fused (see #6052)
• [x] hpx::util::basic_any NO NEED TO MOVE THAT
• [x] hpx::chrono::high_resolution_clock NO NEED
• [x] hpx::chrono::high_resolution_timer NO NEED
• [x] hpx::chrono::steady_time_point NO NEED
• [ ] all the names in hpx::execution namespace LEAVE AS LAST
• [x] hpx::lcos::local::channel
• [x] hpx::util::unlock_guard (see #6086)
• [x] hpx::parallel::v1::reduce_by_key (experimental) (see https://github.com/STEllAR-GROUP/hpx/pull/6100)
• [x] hpx::parallel::v1::sort_by_key (experimental) (see https://github.com/STEllAR-GROUP/hpx/pull/6100)
• [x] all the names in header file task_block.hpp (experimental) (see https://github.com/STEllAR-GROUP/hpx/pull/6126)
• [x] all the names in hpx::functional namespace NO NEED
• [x] task_group (see https://github.com/STEllAR-GROUP/hpx/pull/6136)

More generic changes:

• [ ] Remove our steady_time_point and steady_duration implementations as we have removed support for boost::chrono

[vivekd01]

Suggested changes for the each name. --> hpx::is_bind_expression --> hpx::is_placeholder --> hpx::bind_back --> hpx::mem_fn (already deprecated) --> hpx::invoke, hpx::invoke_r (already deprecated) --> hpx::invoke_fused (already deprecated) --> No need to change --> No need to change --> No need to change --> No need to change --> Leave as the last step --> No need to change --> hpx::unlock_guard (already deprecated) --> hpx::experimental::reduce_by_key (already deprecated) --> hpx::experimental::sort_by_key (already deprecated) --> hpx::experimental::task_block (already deprecated) --> No need to change --> hpx::experimental::task_group

[dimitraka]

Went through the list again, looks like we are only left with 2 points, the steady* and the execution.