Fix clippy::unnecessary_map_or warning

```
error: this `map_or` is redundant
   --> src/duration.rs:637:9
    |
637 |         self.0.map_or(false, |this| this == *other)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use a standard comparison instead: `(self.0 == Some(*other))`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_map_or
    = note: `-D clippy::unnecessary-map-or` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::unnecessary_map_or)]`

error: this `map_or` is redundant
   --> src/instant.rs:163:9
    |
163 |         self.0.map_or(false, |this| this == *other)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use a standard comparison instead: `(self.0 == Some(*other))`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_map_or
```

src/duration.rs

@@ -634,7 +634,7 @@ impl Duration {
 
 impl PartialEq<time::Duration> for Duration {
     fn eq(&self, other: &time::Duration) -> bool {
-        self.0.map_or(false, |this| this == *other)
+        self.0 == Some(*other)
     }
 }
 

src/instant.rs

@@ -160,7 +160,7 @@ impl Instant {
 
 impl PartialEq<time::Instant> for Instant {
     fn eq(&self, other: &time::Instant) -> bool {
-        self.0.map_or(false, |this| this == *other)
+        self.0 == Some(*other)
     }
 }