Use modified async_kernel to extract true backtraces from monitor exns

[mrmr1993]

This PR

• modifies our forked async_kernel to add a Monitor.exn_to_error function
  • this extracts the backtrace from the special hidden Monitor.Exn_for_monitor.t info inside Monitor.Error_ exceptions, wrapping it into an Error.t so our normal JSON-of-Error.t conversion can use it
  • PR at Add Monitor.exn_to_error async_kernel#5
• uses the function to give a more accurate backtrace when crashing/handling toplevel exceptions.

This was inspired by #6901, where the Invalid_curve_point exception is very unlikely to have come from a format.ml backtrace that is shown.

Checklist:

• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them:

[mrmr1993]

CI is very unhappy with this PR. Closing in favour of #7277.