Enable specification of offsets in bytes for large values.

Because the data for a single array can be in a large buffer in CDI-PIO, I created a version of xt_offset_ext that takes MPI_Aint offsets and strides instead of int.

• Reasonably, one could extend xt_aoffset_ext to already use MPI_Count for the size field when available (and simply fail as a first implementation if a count > INT_MAX results).
• A similar implementation would be needed for completeness for xt_redist_p2p_off_new and some other constructors.

I'm creating this merge request for the sake of discussing the issues still to be addressed before a merge can be finalized.