Add tests for size, alignment, trait impls

This ensures that nothing will regress without knowing. As it turns out,
there was a missing implementation that was caught during this. As such,
`impl AddAssign<Duration> for `StdDuration` was added in this commit as
well.

src/duration.rs

@@ -4,7 +4,7 @@ use core::cmp::Ordering;
 use core::convert::{TryFrom, TryInto};
 use core::fmt;
 use core::iter::Sum;
-use core::ops::{Add, Div, Mul, Neg, Sub, SubAssign};
+use core::ops::{Add, AddAssign, Div, Mul, Neg, Sub, SubAssign};
 use core::time::Duration as StdDuration;
 
 use crate::error;
@@ -837,6 +837,15 @@ impl Add<Duration> for StdDuration {
 
 impl_add_assign!(Duration: Self, StdDuration);
 
+impl AddAssign<Duration> for StdDuration {
+    fn add_assign(&mut self, rhs: Duration) {
+        *self = (*self + rhs).try_into().expect(
+            "Cannot represent a resulting duration in std. Try `let x = x + rhs;`, which will \
+             change the type.",
+        );
+    }
+}
+
 impl Neg for Duration {
     type Output = Self;
 

tests/integration/duration.rs

@@ -471,6 +471,17 @@ fn add_assign_std() {
     assert_eq!(duration, 0.seconds());
 }
 
+#[test]
+fn std_add_assign() {
+    let mut duration = 1.std_seconds();
+    duration += 1.seconds();
+    assert_eq!(duration, 2.seconds());
+
+    let mut duration = 500.std_milliseconds();
+    duration += 500.milliseconds();
+    assert_eq!(duration, 1.seconds());
+}
+
 #[test]
 fn neg() {
     assert_eq!(-(1.seconds()), (-1).seconds());

tests/integration/main.rs

@@ -28,7 +28,12 @@
     unused_qualifications,
     variant_size_differences
 )]
-#![allow(clippy::cognitive_complexity, clippy::clone_on_copy, clippy::cmp_owned)]
+#![allow(
+    clippy::clone_on_copy,
+    clippy::cmp_owned,
+    clippy::cognitive_complexity,
+    clippy::missing_const_for_fn
+)]
 
 extern crate quickcheck_dep as quickcheck;
 
@@ -55,6 +60,7 @@ mod format_description;
 mod formatting;
 mod instant;
 mod macros;
+mod meta;
 mod month;
 mod offset_date_time;
 mod parse_format_description;