Env,proto: Expose a dummy WASM `step` function to the protocol

Context

This MR provides the necessary boilerplate to later plug the WASM step function into the PVM machinery. We use this opportunity to introduce a test that shows that we can (1) generate a proof outside of the protocol, and (2) make the verifier of the protocol to verify it. Of course, because the PVM does nothing, the proof is actually completely empty.

Manually testing the MR

This is mostly boilerplate, so there is not much to do. Modifying the step function of the Wasm_2_0_0.Make functor to raise an error can be a solution to convince oneself that the function is actually called.

dune exec src/proto_alpha/lib_protocol/test/integration/main.exe -- test "^sc rollup wasm$"

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR