OCaml solutions don't build with current version of OCaml

[rbergen]

While updating a number of Docker images published on Docker Hub in the context of this project, I found that the OCaml solutions don't build with a current version of OCaml (5.1, to be precise). Two reasons I've been able to identify but also noting that there may be more:

• corebuild is no longer available. After reading some documentation it seems to me that dune is now "the" build system for OCaml, but I may be wrong.
• Core.Unix has been deprecated.

Personally, I am definitely not sufficiently knowledgeable about OCaml to make the modifications that would be required to make the solutions build again.

Without wanting to imply any obligation to look into this by the solutions' author @gkpotter, I'm calling out to him specifically in case he would be willing and able to do so.