Added Ord and Hash as derived traits for tokio::time::Instant

[tsidea]

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.

Motivation

tokio::time::Instant is just a wrapper around std::time::Instant, as such it should
behave just like std::time::Instant. That should include what kind of
traits it derives.

Solution

Just added the same derived traits as std::time::Instant

[katyo]

I also agree that this is pretty useful traits which currently doesn't implemented for Instant.
So when I need get min/max instant I must convert it to std and back, which I would like to avoid.

[LucioFranco]

Thanks!