Changed unwrap_or to unwrap_or_else in some places.

The discussion seems to have resolved that this lint is a bit "noisy" in
that applying it in all places would result in a reduction in
readability.

A few of the trivial functions (like `Path::new`) are fine to leave
outside of closures.

The general rule seems to be that anything that is obviously an
allocation (`Box`, `Vec`, `vec![]`) should be in a closure, even if it
is a 0-sized allocation.

library/std/src/time.rs

@@ -322,7 +322,7 @@ impl Instant {
     /// ```
     #[stable(feature = "checked_duration_since", since = "1.39.0")]
     pub fn saturating_duration_since(&self, earlier: Instant) -> Duration {
-        self.checked_duration_since(earlier).unwrap_or(Duration::new(0, 0))
+        self.checked_duration_since(earlier).unwrap_or_default()
     }
 
     /// Returns the amount of time elapsed since this instant was created.