Scoru: harden the levels processed in a commitment

If you post a commitment at level x, you must not execute the inputs at level x, you need to stop at the last input of x - 1. At least its what is enforced in the proto.

The rollup-node apparently does not respect this behavior. Next actions:

1. Modify Sc_rollup_proof_repr.cut_at_level to include the inputs at level x
2. Test in an proto integration test that a honest players wins the game if the opponent process x-1 or x+1 levels
3. Test in the rollup-node that the commitment includes the correct level (e.g., by looking inside the arith pvm state of a commitment state)