Fix segfault: capture rust heap references before yeilding to OCaml

[mrmr1993]

This PR fixes the segmentation fault reported in #6824.

The 'header' (ie. data pointer + length) of the input vectors were stored in the OCaml heap. This was fine when they were run synchronously, but the async version allows the OCaml GC to run while the function is also running. A badly timed GC can move the header and leave rust's reference pointing at arbitrary data, and dereferencing the vector's data pointer may then result in a segfault.

This PR changes the behaviour to store only a (rust heap) pointer in the OCaml heap. As long as we capture the pointer before yielding to OCaml, we know that we hold a reference to the intended data.

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: