This adds a module that wraps some Bos actions to add a "dry run" mode. The goal is to expose some of these actions and test them at a high level. (this is not intended to implement a --dry-run flag)

The implementation is based on the free monad pattern used in OpamProcess.Job and explained in [this article]. A list of files is passed to the "dry run" evaluator, which is used to keep track of a static approximation of the file system. This is used for example to determine which value is returned by exists.

The (>>=) from Rresult.R is replaced by a new one in Mirage_job, which means that the conversion is pretty straightforward (that code was monadic already).

As for other similar PRs I have opened, I have some questions as to how to make this not impact the API too much:

• where should the jobs be defined? At the moment I just exposed configure behind a (**/**), but this does not feel too great.
• would it make sense to move this to a separate library? Maybe after all similar code has been converted?

Let me know what you think. Thanks!