PartialEq and Eq for the Schedule type about cron

[koenichiwa]

Just combining the following two conversations
#62 (comment)
#71 (comment)

This could be implemented as

impl PartialEq for Schedule {
    fn eq(&self, other: &Schedule) -> bool {
        self.source == other.source
    }
}

pub struct TimeSpecEq(Schedule);

impl PartialEq for TimeSpecEq {
    fn eq(&self, other: &OrdinalsEqSchedule) -> bool{
        self.0.fields == other.0.fields
    }
}

plus the normal derives

I thought OrdinalsEqSchedule was a bit confusing, because Ordinals is not exposed to the user, thats why I would rename it to TimeSpecEq.

This would expose TimeSpecEq if you want to check on just the timespecs. This would be usefull if you want to check if @Weekly is equivalent to 0 0 0 * * 1 * (or the use case in #62)