time: support conversion RFC3339 time dates to unix time

fixes cue-lang#319

pkg/time/testdata/gen.txtar

@@ -6,13 +6,25 @@ import "time"
 t1: time.Time & "1937-01-01T12:00:27.87+00:20"
 t2: time.Time & "no time"
 t3: time.Unix(1500000000, 123456)
+t4: time.ToUnix("2017-07-14T02:40:00.000123456Z")
+t5: time.ToUnix("07-14-2017T02:40:00.000123456Z")
+t6: time.ToUnixNano("2017-07-14T02:40:00.000123456Z")
+t7: time.ToUnixNano("07-14-2017T02:40:00.000123456Z")
 -- out/time --
 Errors:
 t2: invalid value "no time" (does not satisfy time.Time): error in call to time.Time: invalid time "no time":
     ./in.cue:4:17
+error in call to time.ToUnix: invalid time "07-14-2017T02:40:00.000123456Z":
+    ./in.cue:7:5
+error in call to time.ToUnixNano: invalid time "07-14-2017T02:40:00.000123456Z":
+    ./in.cue:9:5
 
 Result:
 t1: "1937-01-01T12:00:27.87+00:20"
 t2: _|_ // t2: invalid value "no time" (does not satisfy time.Time): error in call to time.Time: invalid time "no time"
 t3: "2017-07-14T02:40:00.000123456Z"
+t4: 1500000000
+t5: _|_ // error in call to time.ToUnix: invalid time "07-14-2017T02:40:00.000123456Z"
+t6: 1500000000000123456
+t7: _|_ // error in call to time.ToUnixNano: invalid time "07-14-2017T02:40:00.000123456Z"
 

pkg/time/time.go

@@ -201,3 +201,38 @@ func Unix(sec int64, nsec int64) string {
 	t := time.Unix(sec, nsec)
 	return t.UTC().Format(time.RFC3339Nano)
 }
+
+// Unix returns t as a Unix time, the number of seconds elapsed since January 1,
+// 1970 UTC. The result does not depend on the location associated with t.
+// Unix-like operating systems often record time as a 32-bit count of seconds,
+// but since the method here returns a 64-bit value it is valid for billions of
+// years into the past or future.
+
+// ToUnix returns the provided RFC3339 time-date as a Unix time, the number of
+// seconds elapsed since January 1, 1970 UTC.
+func ToUnix(value string) (int64, error) {
+	t, err := parse(value)
+	if err != nil {
+		return 0, err
+	}
+	return t.Unix(), nil
+}
+
+// ToUnix returns the provided RFC3339 time-date as a Unix time, the number of
+// nanoseconds elapsed since January 1, 1970 UTC. The result is undefined if the
+// Unix time in nanoseconds cannot be represented by an int64 (a date before the
+// year 1678 or after 2262).
+func ToUnixNano(value string) (int64, error) {
+	t, err := parse(value)
+	if err != nil {
+		return 0, err
+	}
+	return t.UnixNano(), nil
+}
+
+func parse(value string) (time.Time, error) {
+	if _, err := Time(value); err != nil {
+		return time.Time{}, err
+	}
+	return time.Parse(time.RFC3339Nano, value)
+}