Add WiP datetime package and tests.

A new package `core:time/datetime` has been added which can represent moments much further in the past and future than `core:time`.
It is based on *the* reference work on the subject, Calendrical Calculations Ultimate Edition, Reingold & Dershowitz.

More procedures will be added to it in the future, to for example calculate the 3rd Thursday in March to figure out holidays.
The package has been tested for more than a year and can handle dates 25 quadrillion years into the past and future with 64-bit day ordinals, or 5 million with 32-bit ones.

This also fixes a longstanding bug where converting between YYYY-MM:DD hh:mm:ss and `time.Time` and back could result in a mismatch.

RFC 3339 timestamps can now also be parsed using the `core:time` package.

core/time/rfc3339.odin

@@ -0,0 +1,122 @@
+package time
+// Parsing RFC 3339 date/time strings into time.Time.
+// See https://www.rfc-editor.org/rfc/rfc3339 for the definition
+
+import dt "core:time/datetime"
+
+// Parses an RFC 3339 string and returns Time in UTC, with any UTC offset applied to it.
+// Only 4-digit years are accepted.
+// Optional pointer to boolean `is_leap` will return `true` if the moment was a leap second.
+// Leap seconds are smeared into 23:59:59.
+rfc3339_to_time_utc :: proc(rfc_datetime: string, is_leap: ^bool = nil) -> (res: Time, consumed: int) {
+	offset: int
+
+	res, offset, consumed = rfc3339_to_time_and_offset(rfc_datetime, is_leap)
+	res._nsec += (i64(-offset) * i64(Minute))
+	return res, consumed
+}
+
+// Parses an RFC 3339 string and returns Time and a UTC offset in minutes.
+// e.g. 1985-04-12T23:20:50.52Z
+// Note: Only 4-digit years are accepted.
+// Optional pointer to boolean `is_leap` will return `true` if the moment was a leap second.
+// Leap seconds are smeared into 23:59:59.
+rfc3339_to_time_and_offset :: proc(rfc_datetime: string, is_leap: ^bool = nil) -> (res: Time, utc_offset: int, consumed: int) {
+	moment, offset, count := rfc3339_to_components(rfc_datetime)
+	if count == 0 {
+		return
+	}
+
+	// Leap second handling
+	if moment.minute == 59 && moment.second == 60 {
+		moment.second = 59
+		if is_leap != nil {
+			is_leap^ = true
+		}
+	}
+
+	if _res, ok := datetime_to_time(moment.year, moment.month, moment.day, moment.hour, moment.minute, moment.second, moment.nano); !ok {
+		return {}, 0, 0
+	} else {
+		return _res, offset, count
+	}
+}
+
+// Parses an RFC 3339 string and returns Time and a UTC offset in minutes.
+// e.g. 1985-04-12T23:20:50.52Z
+// Performs no validation on whether components are valid, e.g. it'll return hour = 25 if that's what it's given
+rfc3339_to_components :: proc(rfc_datetime: string) -> (res: dt.DateTime, utc_offset: int, consumed: int) {
+	count: int
+	moment, offset, ok := _rfc3339_to_components(rfc_datetime, &count)
+	if !ok {
+		return
+	}
+	return moment, offset, count
+}
+
+// Parses an RFC 3339 string and returns datetime.DateTime.
+// Performs no validation on whether components are valid, e.g. it'll return hour = 25 if that's what it's given
+@(private)
+_rfc3339_to_components :: proc(rfc_datetime: string, consume_count: ^int = nil) -> (res: dt.DateTime, utc_offset: int, ok: bool) {
+	// A compliant date is at minimum 20 characters long, e.g. YYYY-MM-DDThh:mm:ssZ
+	(len(rfc_datetime) >= 20) or_return
+
+	// Scan and eat YYYY-MM-DD[Tt]
+	res.year  = scan_digits(rfc_datetime[0:], "-",  4) or_return
+	res.month = scan_digits(rfc_datetime[5:], "-",  2) or_return
+	res.day   = scan_digits(rfc_datetime[8:], "Tt", 2) or_return
+
+	// Scan and eat HH:MM:SS, leave separator
+	res.hour   = scan_digits(rfc_datetime[11:], ":", 2) or_return
+	res.minute = scan_digits(rfc_datetime[14:], ":", 2) or_return
+	res.second = scan_digits(rfc_datetime[17:], "",  2) or_return
+	count := 19
+
+	if rfc_datetime[count] == '.' {
+		// Scan hundredths. The string must be at least 4 bytes long (.hhZ)
+		(len(rfc_datetime[count:]) >= 4) or_return
+		hundredths := scan_digits(rfc_datetime[count+1:], "", 2) or_return
+		count += 3
+
+		res.nano = 10_000_000 * hundredths
+	}
+
+	// Scan UTC offset
+	switch rfc_datetime[count] {
+	case 'Z':
+		utc_offset = 0
+		count += 1
+	case '+', '-':
+		(len(rfc_datetime[count:]) >= 6) or_return
+		offset_hour   := scan_digits(rfc_datetime[count+1:], ":", 2) or_return
+		offset_minute := scan_digits(rfc_datetime[count+4:], "",  2) or_return
+
+		utc_offset = 60 * offset_hour + offset_minute
+		utc_offset *= -1 if rfc_datetime[count] == '-' else 1
+		count += 6
+	}
+
+	if consume_count != nil {
+		consume_count^ = count
+	}
+	return res, utc_offset, true
+}
+
+@(private)
+scan_digits :: proc(s: string, sep: string, count: int) -> (res: int, ok: bool) {
+	needed := count + min(1, len(sep))
+	(len(s) >= needed) or_return
+
+	#no_bounds_check for i in 0..<count {
+		if v := s[i]; v >= '0' && v <= '9' {
+			res = res * 10 + int(v - '0')
+		} else {
+			return 0, false
+		}
+	}
+	found_sep := len(sep) == 0
+	#no_bounds_check for v in sep {
+		found_sep |= rune(s[count]) == v
+	}
+	return res, found_sep
+}