Implement Sum and release a new version (#53)

This change implements the `Sum` traits so that iterators of `Au` can be
summed. We would like to do this in Servo. In addition, it also prepares
for a new release of the crate, so this can be used.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "app_units"
-version = "0.7.3"
+version = "0.7.4"
 authors = ["The Servo Project Developers"]
 description = "Servo app units type (Au)"
 documentation = "https://docs.rs/app_units/"

src/app_unit.rs

@@ -8,6 +8,7 @@ use num_traits::Zero;
 use serde::{Serialize, Deserialize, Deserializer};
 
 use std::{fmt, i32, default::Default, ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Neg, Rem, Sub, SubAssign}};
+use std::iter::Sum;
 
 /// The number of app units in a pixel.
 pub const AU_PER_PX: i32 = 60;
@@ -178,6 +179,12 @@ impl DivAssign<i32> for Au {
     }
 }
 
+impl<'a> Sum<&'a Self> for Au {
+    fn sum<I: Iterator<Item = &'a Self>>(iter: I) -> Self {
+        iter.fold(Self::zero(), |a, b| a + *b)
+    }
+}
+
 impl Au {
     /// FIXME(pcwalton): Workaround for lack of cross crate inlining of newtype structs!
     #[inline]