[Mpi3-bwcompat] MPI_Size and non-count int parameters

[Fab Tillier]

Should MPI_Size be used where one would likely use size_t?

For example, MPI_Type_create_struct (page 86), should the count parameter be MPI_Size?  Likewise, should the three integers output by MPI_Type_get_envelope (page 105) be MPI_Size?

Or is this yet another distinct type, and if so, should we try to cover it in this ticket?

Thanks,
-Fab