Reintroduce generic artifacts

[andrey-kuprianov]

We've had an initial attempt of introducing artifacts, which have then been deleted in favor of a simpler approach.

Artifacts are meant as a generic representation of (intermediate) stages of processing; each should come with the clean description of the format, and with the ability to (de-)serialise to/from files or strings in a generic way.

Current artifacts include:

• TLA file
• Cfg file
• TLA trace
• JSON trace

Currently, e.g. TLA and Cfg files are represented as references to filesystem paths, which makes it really hard to operate them independently of that. Instead each artifact should know only it's content, no matter where it comes from, and be able to show/save it upon request. This will make it transparent to integrate into the modules, and further into the CLI.

What to do:

• Bring back (and may be rework/extend) the Artifact trait
• Describe clearly what is the format of the current artifacts (related to Reliable extraction of counterexample traces #58)
• Refactor the current artifacts to implement that trait

[andrey-kuprianov]

Closed via #70 and #80