Add handle serialization routines

[dalcinl]

No description provided.

[dalcinl]

@hzhou We need these new routines in MPICH.

[dalcinl]

@jeffhammond In the new MPI std document we have "It is erroneous to call this function with an invalid handle argument". However, these routines do not follow the usual "return error code" pattern, and therefore is not clear at all how error handling should be implemented and eventually handled by users. IMHO, this should be amended following the model of c2f/f2c routines, that is, a) null handles can be properly mapped back and forth, but invalid MPI handles are mapped to invalid C int handles and vice versa. If an MPI implementation can afford the overhead of distinguishing valid vs invalid handles via some internal mechanism, then the 0 for the C int value and NULL (e.g. for MPI_Comm in the ABI) would be great choices for returning invalid handles (at least within the new ABI).

[jeffhammond]

We say it is erroneous to place the burden of checking entirely on the user, to allow trivial implementations.

[dalcinl]

We say it is erroneous to place the burden of checking entirely on the user, to allow trivial implementations.

OK, I clearly misunderstood the actual meaning.

[dalcinl]

@jeffhammond I think this one is also ready for merge.

[jeffhammond]

Does this remove all f2c/c2f?

[dalcinl]

Does this remove all f2c/c2f?

No, not at all! Should it??

[dalcinl]

@jeffhammond I brought your commit from #48 to update this PR. Now the mpi4py build test pass, although with a branch other than master.

If everything is good with this PR from your side, please approve.
Leave the merge to me such that I can address the testing part.