Adding missing methods to Instant, matching Rust 1.39

Author: fusetim

src/wasm.rs

@@ -70,6 +70,26 @@ impl Instant {
     pub fn elapsed(&self) -> Duration {
         Instant::now() - *self
     }
+
+    pub fn checked_duration_since(&self, earlier: Instant) -> Option<Duration> {
+        let since = self.duration_since(earlier);
+        if since >= 0 {
+            return Some(Since);
+        }
+        None
+    }
+
+    pub fn saturating_duration_since(&self, earlier: Instant) -> Duration {
+        self.checked_duration_since(earlier).unwrap_or_default()
+    }
+
+    pub fn checked_add(&self, duration: Duration) -> Option<Instant> {
+        Some(*self + duration)
+    }
+
+    pub fn checked_sub(&self, duration: Duration) -> Option<Instant> {
+        Some(*self - duration)
+    }
 }
 
 impl Add<Duration> for Instant {