Multicoretests-CI bot for ocaml/ocaml developers

[benmandrew]

A possible end-goal of ocaml-multicore/multicoretests is for it to become a test-suite used by OCaml compiler developers for checking PRs. One way to do this would be through a bot attached to the ocaml/ocaml repo that can be invoked to run the expensive test-suite when it is deemed appropriate, in the same way that the Coq bot works (coq/bot).

This could use parts of Multicoretests-CI for the infrastructure that runs the tests on multiple platforms.

[jmid]

Cc to @shakthimaan as he is preparing work on a bot for https://github.com/ocaml-bench/sandmark

Ideally, we should

• offer the ocaml compiler devs just one bot with a generic name, say ocamlbot, rather than have them memorize 3 different ones, and
• make it as convenient as possible for them to use.

One possibility is to trigger actions by tagging the bot (and accepting sub-commands): @ocamlbot sandmark (or benchmark), @ocamlbot multicoretests (or stresstest`).

Another possibility is to trigger actions by recognizing labels (with similar names to the above).