Implement PartialOrd on Duration

[wcarmon]

allow duration comparisons (similar to chrono::TimeDelta, which has the same structure)

https://docs.rs/chrono/latest/src/chrono/time_delta.rs.html#52

[caspermeijn]

I like the idea.

1. protobuf.rs is a generated file. When the tests are run, it will be overwritten. This could be fixed by moving the derive to here:

   prost/tests/src/bootstrap.rs

   Line 42 in 8d4cac5

   .btree_map(["."])

   
   Then running the tests will generate a new version with your derive.
2. Duration is not guaranteed to be normalized. That means that nanos field could be larger than 1 second and the ordering would be off. I am unsure how big this problem is, as the documentation states that it should be within range. Let's ignore normalization for now.
3. I would like to see some tests to confirm the ordering works as expected. As I understand the documentation of derive(Ord) it sorts based on the alphabetical order, which would be incorrect in this case: https://dev-doc.rust-lang.org/stable/std/cmp/trait.PartialOrd.html#derivable
4. Are you aware of this earlier work? Maybe you could take that as inspiration: Feat: Implements ord and partialord to timestamp #995