WIP: Add coq-of-ocaml to the vendors

Our goal is to add coq-of-ocaml to the vendors, so that we can keep track of the latest changes of coq-of-ocaml for the CI.

What we do:

1. First commit:

• we add the sources of coq-of-ocaml in the vendors/ folder;
• we modify the Makefile to also build coq-of-ocaml;
• we change the CI rule for coq:lint to run the locally built version of coq-of-ocaml;

2. Second commit:

• we black-list some errors so that the coq:lint succeeds. Other MRs will help to reduce this black-list, we can go step by step and see the progress.