WASM/PVM: expose `compute_step_many` for future usage in the L2 node

Context

Now that we know compute_step_many ~max_steps is strictly equivalent to compute_step (.. <max_steps> (compute_step tree)), we can expose it safely to be used in an L2 node, instead of making it an test function only.

Fixes #3800 (closed)

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Checklist

• Document the interface of any function added or modified (see the coding guidelines)
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• Provide automatic testing (see the testing guide).
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