Add `yearly_simple_date` collector that detects simple yearly
recurrences (`RRULE:FREQ=YEARLY` with no extra constraints) and stores
the month/day pair in a new `simple_yearly` field on `Remind.rem`.
When rendering, events with `simple_yearly` set are emitted as
`REM <Month> <Day> MSG <summary>` instead of the full dated form.
Add `month_of_int` and `string_of_month` helpers in `Utils`, and update
the RRULE dataset comment with UIDs for easier debugging.
Paolo Donadeo
33dee76b4c