This MR introduces the first part of refactored Prosa's implementation folder, namely a generic function for computing valid work-conserving uniprocessor schedules for given priority policies, readiness models, and preemption policies.

This will be useful for future assumption-free instantiations of analyses in Prosa.

@mmaida @sbozhko: please review. In particular, please check out the two new readiness model classes nonclairvoyant_readiness and valid_nonpreemptive_readiness.