Using multicore-magic-dscheck for dscheck tests

[lyrm]

This PR fixes the issue with running DScheck tests on data structures using multicore-magic Transparent_atomic and/or Atomic_array modules. They both need do have their atomic calls shadowed by DScheck's atomics and until now, we were using some working but not generic tricks. Especially, this had some visible consequences on the saturn library itself (with a transparent_atomic module that only had that use).

TODO :

• Add the proper dependencies to multicore-magic-dscheck and DScheck when their releases.

[polytypic]

This looks nice! 👍

[polytypic]

We obviously need to get rid of the pin-depends, but otherwise this looks very good. So, I'm marking this as approved. 👍

It will probably take some time to go through the opam publish manoeuvers twice (for dscheck and multicore-magic).