Pad output of `Display` on `Instant`

Zero-pad the milliseconds part of a displayed `Instant` to include
three leading zeros.
Closes: #297
Approved by: whitequark

src/time.rs

@@ -168,7 +168,7 @@ impl Duration {
 
 impl fmt::Display for Duration {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{}.{}s", self.secs(), self.millis())
+        write!(f, "{}.{:03}s", self.secs(), self.millis())
     }
 }