OCaml 5.0 compatibility

[kLabz]

Makes the codebase compatible with both ocaml 4.08 and 5.0. CI now runs for both on linux.

I had to disable StdThread.kill on eval which was probably not even working in the first place. Will likely need to disable it more nicely (from std side?) and cleanup the commented out code.

Closes #11271

[andyli]

Would you rebase it on top of 4.3.2 such that I can use it as a patch for packaging?

[kLabz]

I can't until I have a new implementation for the memory-related features :/

[andyli]

I thought you've disabled the memory features temporily in this PR?
It would be nice to have 4.3.2 packages even if the memory features are disabled (I guess they're not critical).
To be clear, I'm not asking for merging into development, but keeping a seperate ocaml-5-ready branch/PR for the 4.3_bugfix branch.

[kLabz]

I updated the lib we needed for memory features so that it works on both ocaml 4.08 and 5.0; I didn't want 4.3.x releases missing features that would break some editor features.

I'm not sure what exactly you need. If this PR produces what you're after, I can backport it to 4.3_bugfix branch tomorrow morning.

[andyli]

Great! Thanks a lot for the quick work.

backport it to 4.3_bugfix branch

Yes, that will do. I will than able to generate a patch to apply it on top of the 4.3.2 source.

[kLabz]

I also pushed to 4.3_bugfix (after checking CI on a separate branch), you'll have to wait a couple hours for CI to complete (arm64.. :/) if you need it otherwise you should be good to go.