Share some code between QCheck and QCheck2

[vch9]

Well, Yes, we copied pasted QCheck to QCheck2, but that's be better if we could share few parts of the code. For instance there is the low hanging-fruit Print module that should be the same in both qchecks. In situation like https://github.com/c-cube/qcheck/pull/245 we end up copy-pasting code for no good reason.