derive Ord and Eq for Epoch

[mkolopanis]

Derives Ord and Eq for epoch.
I also added a small test to check expected orderings between two epochs.

[ChristopherRabotin]

Looks good, thanks!
For the std cmp ordering in the test, you can use core::cmp::ordering if the std feature is not available. That change would allow the tests to check the equality even without std.

[mkolopanis]

Thanks for the tip, always forget about core counterparts

[ChristopherRabotin]

Thanks for the PR! I'll release this with version 3.3 this weekend if that works for you too.