Memory corruption fixes

[mrmr1993]

This PR

• removes the optimised, no-double-copy code for converting Vec<A> to ocaml::Array<B> in rust
• replaces the uses with an explicit map Vec<A> -> Vec<B> and falls back on the implicit Vec<B> -> ocaml::Array<B> conversion

This appears to fix a memory corruption issue, where a bad allocation for a polynomial commitment could cause a Fatal error: out of memory message. The tool I wrote to find the bug would consistently trigger this in < 5000 iterations on the old version; the new version still hasn't after 200,000 (test still running).

I'm not clear on why this fixes the issue: the underlying logic for the two versions is nearly identical, save for an extra (presumably unnecessary) vector allocation in the new version. However, in the old version the compiler seems to optimise away some writes that are needed to tell the OCaml GC about some live heap memory; in the new version these writes appear happen successfully.

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:

This fixes #6674