Draft: Multicore compatibility

Context

Adds an option to build tezos with Multicore OCaml compiler. Multicore compiler variant 4.12.0+domains is added in opam-repository!202 (closed). This would enable building tezos with Multicore OCaml. We have tested this on our fork at https://github.com/ocaml-multicore/tezos and so far tezos seems to work with Multicore without any major changes!

We had hardcoded the compiler version on our fork for the sake of testing. In this MR I propose to add an option to build with Multicore with build-multicore-deps. I'm open to hearing any suggestions on making this even simpler.

Manually testing the MR

Reviewers can build the Multicore version with:

make build-multicore-deps # Installs a 4.12.0+domains compiler switch
make
make test-unit # The tests should pass

flextesa tests are known to fail on Multicore switch, full log can be accessed here.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR