Remove an unecessary lifetime in the Sum implementation (#56)

src/app_unit.rs

@@ -187,7 +187,7 @@ impl<'a> Sum<&'a Self> for Au {
     }
 }
 
-impl<'a> Sum<Self> for Au {
+impl Sum<Self> for Au {
     fn sum<I: Iterator<Item = Self>>(iter: I) -> Self {
         iter.fold(Self::zero(), |a, b| a + b)
     }