Add debug keep_vcs: allow to use a given filename to save prover files.
It is now possible to give an optional argument to schedule_proof_attempt so that (when in debug mode) the file given is used to store the generated input file of the prover. In non debug mode, the file given to schedule_proof_attempt is used but removed after the call to the prover. Currently, no file is ever given to schedule_proof_attempt but this can be used by people using Why3 as a backend.
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