0.23

• Quote and escape in Print.string and Print.char in the QCheck module, mirroring the QCheck2.Print module's behaviour. Also quote and escape Print.bytes in both QCheck and QCheck2.
• Clean-up QCheck and QCheck2 documentation pages
• Add exponential generator to QCheck, QCheck.Gen, and QCheck2.Gen
• Add Shrink.bool and use it in QCheck.bool
• Remove unread fun_gen field from QCheck2's fun_repr_tbl type thereby silencing a compiler warning