fix WriteRef

[batatovich]

Summary

Fix WriteRef. When a callee performed *ref = new_value on a mutable reference to a struct, enum, or vector, the new data was written to a fresh heap allocation but the caller's reference still pointed to the old allocation, so the caller never observed the update

Changes Made

• Now WriteRef overrides the original reference with the new value

Checklist

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• I have added/updated tests where appropriate.
• I have run relevant tests locally.
• I have updated relevant documentation (if applicable).
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