ocaml: fix compatibility with ctypes 0.21.0

[emillon]

In ctypes < 0.21.0, the ctypes and ctypes.stubs libraries were installed in the same directory, so depending on one would make the other one visible.
ctypes 0.21.0 installs them in different directories so this makes it an error.
hacl-star-raw uses ctypes.stubs so the dependency should be recorded.

[cla-bot]

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[emillon]

@cryspen/core I agree with the terms of the CLA.

[franziskuskiefer]

Thanks! lgtm but @victor-dumitrescu should have a look as well as maintainer of the package.

@victor-dumitrescu can you have a look and bump versions/do a new release if needed?

[coveralls]

Pull Request Test Coverage Report for Build 5596166449

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 52.764%

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Totals
Change from base Build 5596163700:	0.02%
Covered Lines:	28938
Relevant Lines:	54844

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💛 - Coveralls

[victor-dumitrescu]

Hi, thanks for submitting the fix! @emillon can you confirm that you tested it with ctypes 0.21.0? Do we need to add a dependency to our opam file too?

[emillon]

can you confirm that you tested it with ctypes 0.21.0?

Yes, I have tested that with ctypes 0.21.0. To be more precise about the problem, hacl-star-raw main build and installs correctly even with ctypes 0.21.0, but then hacl-star does not build correctly. hacl-star-raw built with this fix will build and install correctly, and be usable when used with hacl-star.

Do we need to add a dependency to our opam file too?

In terms of metadata:

• all released versions of hacl-star-raw require ctypes < 0.21.0 (I updated the opam-repository when releasing ctypes 0.21.0).
• the future version released with this PR is compatible with < 0.21.0 and >= 0.21.0. So the opam file in here does not need to be modified.

[victor-dumitrescu]

I see, I was wondering whether I should add an explicit extra dependency in the opam file on ctypes.foreign for example, or if it's ok to only have ctypes

[emillon]

ctypes has 2 APIs: stub generation (generate C and ml code from descriptions) and foreign (use libffi to call directly into a .so file). If you use the ctypes.foreign library, you need in addition to record the dependency on the ctypes-foreign package, but that's not the case of hacl-star-raw.