time: turn Time.unix to i64, so it can represent times before 1970-01-01, fix time operators, add more tests (#11050)

vlib/time/time_addition_test.v

@@ -0,0 +1,33 @@
+import time
+
+fn test_add_to_day_in_the_previous_century() ? {
+	a := time.parse_iso8601('1900-01-01') ?
+	aa := a.add_days(180)
+	dump(a.debug())
+	dump(aa.debug())
+	assert aa.ymmdd() == '1900-06-29'
+}
+
+fn test_add_to_day_in_the_past() ? {
+	a := time.parse_iso8601('1990-03-01') ?
+	aa := a.add_days(180)
+	assert aa.ymmdd() == '1990-08-27'
+}
+
+fn test_add_to_day_in_the_recent_past() ? {
+	a := time.parse_iso8601('2021-03-01') ?
+	aa := a.add_days(180)
+	assert aa.ymmdd() == '2021-08-28'
+}
+
+fn test_add_to_day_in_the_future_1() ? {
+	a := time.parse_iso8601('3000-11-01') ?
+	aa := a.add_days(180)
+	assert aa.ymmdd() == '3001-04-30'
+}
+
+fn test_add_to_day_in_the_future_2() ? {
+	a := time.parse_iso8601('3000-12-30') ?
+	aa := a.add_days(180)
+	assert aa.ymmdd() == '3001-06-28'
+}