semgrep-core segfault on stack overflow on linux: upgrade ocaml as soon as practical

[mjambon]

A bug that was causing segmentation faults instead of a clean Stack_overflow exception was just fixed in ocaml/ocaml#10633. This bug affects semgrep on linux (x86_64).

Impact: This makes stack overflows a possible source of segfaults on linux. On darwin, it is know that we still get segfaults on stack overflows.

The solution for us is to upgrade ocaml to a version that includes the fix. Semgrep-core for linux is built in docker on an alpine image updated weekly by https://github.com/returntocorp/ocaml-layer, so this is the project to modify to get the fix in.

[mjambon]

A crash was obtained outside of semgrep-core on a very simple test program that just uses alcotest and a test that exhausts stack space. The full report is here: ocaml/ocaml#10645

[nbrahms]

Should be fixed in Semgrep 0.70.0