Changing sub-name sequencing for experimental namespace

[hkaiser]

This change will simplify pulling things from the experimental namespace into the main namespace down the road as it makes different things in the experimental namespace more granular. I.e. if we at some point want to start the process of moving the facilities that are now in namespace hpx::experimental::mpi into the main namespace, this can be easiliy done with:

#if defined(HPX_EXPERIMENTAL_MPI_COMPATIBILITY
namespace hpx { namespace mpi {
    using namespace hpx::experimental::mpi;
}}
#endif

This wouldn't be possible with the current naming scheme.

[hkaiser]

After walking around the block I'm not sure anymore what I was thinking. The current scheme is perfectly fine and can be used in a similar way as outlined above. Closing...

[msimberg]

You had me thinking real hard with this one. That said, if someone thinks that hpx::experimental::mpi actually feels better than hpx::mpi::experimental I think we can go with that as well. The argument for the latter is mostly for consistency with detail, but that doesn't need to stay that way either...