core/stats: report datetime of first item in addition to last

quite useful for quickly determining time span of a data source

my/core/stats.py

@@ -37,6 +37,7 @@ def test_guess_stats() -> None:
     r = res['iter_data']
     assert r == {
         'count': 9,
+        'first': datetime(2020, 1, 1, 1, 1, 1),
         'last': datetime(2020, 1, 3, 1, 1, 1),
     }