Added Ord and Hash as derived traits for tokio::time::Instant (#2239)

The added derived traits mirror the traits of std::time::Instant. As
tokio::time::Instant is just a wrapper around std::time::Instant, it
should also derive all the traits std::time::Instant derives.

tokio/src/time/instant.rs

@@ -6,7 +6,7 @@ use std::time::Duration;
 
 /// A measurement of the system clock, useful for talking to
 /// external entities like the file system or other processes.
-#[derive(Clone, Copy, Eq, PartialEq, PartialOrd)]
+#[derive(Clone, Copy, Eq, PartialEq, PartialOrd, Ord, Hash)]
 pub struct Instant {
     std: std::time::Instant,
 }