New constraints: `earlyStart`, `lateStart`, `earlyFinish`, `lateFinish`

[hughgrigg]

It might be useful to have some business time constraints along the lines of earlyStart, lateStart, earlyFinish, lateFinish.

These would take an exact timestamp, i.e. for a single date and time only, and act as a constraint accordingly.

E.g. new earlyFinish('2019-07-03 13:00') rejects any time after 13:00 but only on 2019-07-03. Similar logic for the other three.

This would make it more natural to add single adjustments to start and finish times.

Currently you'd have to do something like new NotConstraint(new Dates('2019-07-03').andAlso(new BetweenHoursOfDay('13', '00'))) which works OK but is not intuitive at all.

[hughgrigg]

This could be made more useful by having the constructor take the time of day as the first argument, and then any number of constraints as the rest.

E.g. new earlyFinish('13:00', new Dates('2019-07-03')), so the first argument would probably be typed as HourOfDay. Then you could build these constraints flexibly.