[WIP] Add a cache to the call of provers
Design:
- A directory maps filename to the data:
- filename: the checksum of the printed file, the prover information, the command line option given to the prover.
- the data: the output of the prover (what is return by the why3 job server)
Goals:
- use it in CI so that gitlab CI doesn't have to run provers, only nightly will check that the committed cache information is correct.
- Help when writing incrementally proof (trying something, undoing, redoing, ...)
Configuration for:
- using or not (default) the cache
- calling (default) or not the provers in case of cache miss.
- the directory used for the cache.