Integrate Eio's CTF tracing with OCaml's tracing

[talex5]

Eio's Ctf module provides a ring-buffer for storing trace events. Currently, this only works for a single domain. OCaml has its own CTF-based eventlog system (https://ocaml.org/manual/instrumented-runtime.html) and we should probably just use that somehow. It might not currently support user-defined events. There's also a proposal to replace it with eventring (ocaml-multicore/ocaml-multicore#793), which says:

This implementation is designed to be open for extension to user tracing events in the future, as well as integration with statmemprof (e.g continuous allocation profiling).

/cc @sadiqj

[talex5]

@sadiqj and I discussed a couple of ways of doing this today:

• eio could watch eventring to get GC events and then copy them to its own ring, or
• eventring could allow users to add custom events and we store everything there.

The second option seems better, because:

• Having a single ring means events get recorded in the right order.
• It saves us a lot of code managing CTF buffers.
• It allows other tools to add extra events of their own.

The default eventring size is quite small (64KB) but is configurable. Eio uses 1MB by default.

[TheLortex]

Custom event rings PR: ocaml/ocaml#11474
We should make sure that it fits Eio's requirements.

[patricoferris]

Had a quick go at using custom runtime events: patricoferris@671a5b4#diff-b48e492abda9e349dd9f1df758814b45a11c5a63fcd89d65d09dc0bb7194125b -- looks good @TheLortex, but I didn't test it fully.

(note: I had to do some questionable changes to your OCaml PR to get it running on macOS)

[talex5]

PR: #554

[talex5]

This was done in #635.