Implement async module

[rodriados]

Currently, there are no wrappers the non-blocking "asynchronous" MPI functions such as MPI_Ibarrier, MPI_Isend, MPI_Irecv and several others. The wrappers for these functions shall live in the async namespace and return an instance of async::promise.

Asynchronous promises must be waited by using one of the functions async::wait, async::wait_all or async::wait_any. Further details on the functions' and structures' interfaces are yet to be defined.

[rodriados]

Apparently, there is not much use cases for async operations. Therefore, this feature implementation will be postponed to a later version.

[rodriados]

First try implementing this feature can be found on commit 520bffe.
Some problems arise from the current implementation such as mpi::async::waitany cannot know its return type if the same interface from mpi::async::wait and mpi::async::waitall is used. An alternative must be found.