[C++] Implement datediff kernel

Add a kernel to compute the number of years, months, weeks, days, hours, minutes, (micro/milli/nano)seconds, or quarters between two timestamps.

This should act like SQL's DATEDIFF (SQL Server). Pandas doesn't have a convenient equivalent except in the case of days (pd.Timedelta.days) but it can be calculated using Timestamp.to_period.

We have hinnant's date library vendored and this should hopefully be implementable with that.