Add the following date functions to stdlib.trefoil
- date (struct): Represents a specific date with fields year, month, and day.
- is-leap-year?: Determines whether a given year is a leap year.
- days-in-month: Returns the number of days in a given month.
- add-days: Adds a specified number of days to a date.
- first-day: Returns the first day of the month for a given date.
- last-day: Returns the last day of the month for a given date.
- add-days-to-dates: Adds a specific number of days to each date in a list of dates.
Do we require any new language features?