Unpin `runtime_events_tools` and pin `ppxlib` to a version compatible with OCaml 5.1 and 5.2

[punchagan]

ppxlib has a trunk-support branch which is just slightly out of date with the current trunk, and should soon be updated. Once that happens, we should start using that version of ppxlib and the latest version of runtime_events_tools.

This issue is a follow-up issue that supersedes #450 and #451

[punchagan]

ppxlib is now compatible with 5.1 and 5.2

[tmcgilchrist]

https://github.com/tarides/runtime_events_tools/releases/tag/0.5.1 is compatible with 5.1