Implement formula evaluation for DATE statement

src/eval/range.rs

@@ -40,6 +40,11 @@ impl DateRange {
         self.until
     }
 
+    pub fn days(&self) -> impl Iterator<Item = NaiveDate> {
+        (self.from.num_days_from_ce()..=self.until.num_days_from_ce())
+            .map(|n| NaiveDate::from_num_days_from_ce(n))
+    }
+
     pub fn years(&self) -> RangeInclusive<i32> {
         self.from.year()..=self.until.year()
     }