expose Fortran conversions

[simonbyrne]

It would be helpful to expose an interface to convert Fortran handles (i.e. integers) to Julia ones, and vice-versa.

[giordano]

What's a concrete example of what you'd like to do?

[simonbyrne]

I'm reviewing a paper (JuliaCon/proceedings-review#134) where they call Julia from Fortran. See section 3.4 of
https://raw.githubusercontent.com/JuliaCon/proceedings-papers/jcon.00134/jcon.00134/10.21105.jcon.00134.pdf

[simonbyrne]

One of the challenges is that if a user converts c2f -> f2c, they may end up with a different Julia object corresponding to the MPI handle.

I think the way to do this:

• expose f2c/c2f handle conversions (add c2f/f2c handle conversions #798)
• add attribute support to all handle types (currently we only have them defined for Datatypes)
• store the pointer to the Julia object as a pointer to each handle.

[johnomotani]

I would also like this feature. I want to be able to pass a communicator to MUMPS.jl, and MUMPS expects a Fortran communicator. See also https://discourse.julialang.org/t/can-i-pass-mpi-jl-communicator-to-the-fortran-side/76547/2.