Cron data structure and Attoparsec parser. The idea is to embed it in larger
systems which want to roll their own scheduled tasks in a format that people
are used to. 'System.Cron' is where all the interesting datatypes live.
You will also find 'scheduleMatches', which you can use to compare a time
against a 'CronSchedule' to see if an action needs to be performed.
System.Cron.Parser is where you will find the parsers
cronTab. To parse individual schedules up to full crontab