Add #[track_caller] to Duration Div impl

Previously the location of the divide-by-zero error condition would be
attributed to the code in the rust standard library, eg:

	thread 'main' panicked at /home/user/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/time.rs:1172:31:
	divide by zero error when dividing duration by scalar

With #[track_caller] the error is correctly attributed to the callee.

Author: petertodd

library/core/src/time.rs

@@ -1168,6 +1168,7 @@ impl Div<u32> for Duration {
     type Output = Duration;
 
     #[inline]
+    #[track_caller]
     fn div(self, rhs: u32) -> Duration {
         self.checked_div(rhs).expect("divide by zero error when dividing duration by scalar")
     }
@@ -1176,6 +1177,7 @@ impl Div<u32> for Duration {
 #[stable(feature = "time_augmented_assignment", since = "1.9.0")]
 impl DivAssign<u32> for Duration {
     #[inline]
+    #[track_caller]
     fn div_assign(&mut self, rhs: u32) {
         *self = *self / rhs;
     }