Add LocalDate.daysBetween

[mlms13]

daysBetween(a, b) should return an int number of days. Adding that number of days to a should yield b, e.g.

LocalDate.(
  daysBetween(a, b)
  |> addDays(_, a)
  |> eq(b) //true
);

If the first value is less than the second value, the returned result should be positive (e.g. daysBetween(yesterday, today) == 1.

Among other things, this could be useful for determining how far into a year a given date is (as a percentage):

let percentOfYear = ymd => {
  let beginningOfYear = LocalDate.(ymd |> setMonth(Jan) |> setDay(1));
  let between = LocalDate.daysBetween(beginningOfYear, ymd);
  Int.toFloat(between) /. Int.toFloat(LocalDate.daysInYear(ymd));
};

[mlms13]

That daysBetween(beginningOfYear, ymd) stuff should be extracted out into its own function, once we have this. Calculating the "day of year" for a LocalDate seems useful.