Write functions that translate descriptors

[rouson]

Gfortran developer Paul Thomas has outlined the following steps:

• [ ] Write the interface tobind(C) procedures so that gfortran descriptors are translated to C-interop and back.
• [ ] Ensure that the interface is as optimal as possible.
• [ ] Ensure that the C-interop includes are part of gcc's.

@scrasmussen, we probably need some clarification on some of the above items, but this is a starting point. I'm guessing that the first item means to write a function that takes a gfortran descriptor and returns the Fortran 2018 CFI descriptor and a function that is the inverse of the former function.

[rouson]

@ktras is working on these functions, but the result will most likely be part of a much larger commit that also includes updates to ISO_Fortran_binding.c and additional tests. I don't think we need this issue for tracking purposes anymore so I'm closing it. If it makes sense we can include it as one checkbox in a new issue summarizing the expected larger commit.