[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.