Move `SubAssign<OldDuration> next to `Sub<OldDuration`

src/naive/date.rs

@@ -1378,6 +1378,13 @@ impl Sub<OldDuration> for NaiveDate {
     }
 }
 
+impl SubAssign<OldDuration> for NaiveDate {
+    #[inline]
+    fn sub_assign(&mut self, rhs: OldDuration) {
+        *self = self.sub(rhs);
+    }
+}
+
 /// Subtracts another `NaiveDate` from the current date.
 /// Returns a `Duration` of integral numbers.
 /// 
@@ -1414,13 +1421,6 @@ impl Sub<NaiveDate> for NaiveDate {
     }
 }
 
-impl SubAssign<OldDuration> for NaiveDate {
-    #[inline]
-    fn sub_assign(&mut self, rhs: OldDuration) {
-        *self = self.sub(rhs);
-    }
-}
-
 /// The `Debug` output of the naive date `d` is same to
 /// [`d.format("%Y-%m-%d")`](../format/strftime/index.html).
 ///

src/naive/datetime.rs

@@ -1272,6 +1272,13 @@ impl Sub<OldDuration> for NaiveDateTime {
     }
 }
 
+impl SubAssign<OldDuration> for NaiveDateTime {
+    #[inline]
+    fn sub_assign(&mut self, rhs: OldDuration) {
+        *self = self.sub(rhs);
+    }
+}
+
 /// Subtracts another `NaiveDateTime` from the current date and time.
 /// This does not overflow or underflow at all.
 ///
@@ -1327,13 +1334,6 @@ impl Sub<NaiveDateTime> for NaiveDateTime {
     }
 }
 
-impl SubAssign<OldDuration> for NaiveDateTime {
-    #[inline]
-    fn sub_assign(&mut self, rhs: OldDuration) {
-        *self = self.sub(rhs);
-    }
-}
-
 /// The `Debug` output of the naive date and time `dt` is same to
 /// [`dt.format("%Y-%m-%dT%H:%M:%S%.f")`](../format/strftime/index.html).
 ///

src/naive/time.rs

@@ -1126,6 +1126,13 @@ impl Sub<OldDuration> for NaiveTime {
     }
 }
 
+impl SubAssign<OldDuration> for NaiveTime {
+    #[inline]
+    fn sub_assign(&mut self, rhs: OldDuration) {
+        *self = self.sub(rhs);
+    }
+}
+
 /// Subtracts another `NaiveTime` from the current time.
 /// Returns a `Duration` within +/- 1 day.
 /// This does not overflow or underflow at all.
@@ -1186,13 +1193,6 @@ impl Sub<NaiveTime> for NaiveTime {
     }
 }
 
-impl SubAssign<OldDuration> for NaiveTime {
-    #[inline]
-    fn sub_assign(&mut self, rhs: OldDuration) {
-        *self = self.sub(rhs);
-    }
-}
-
 /// The `Debug` output of the naive time `t` is same to
 /// [`t.format("%H:%M:%S%.f")`](../format/strftime/index.html).
 ///