Impl (De)Serialize for SystemTime when serde feature is enabled

[Ekleog]

This is #24 with the last review comments handled.

I did not comment on each replacement of serde's tri! macro with the ? operator.

Kudos to @Fedeparma74 for doing the actual work here, I basically didn't do anything here besides from handling the very few comments that were left and fixing CI :)

Closes #24

[Ekleog]

@daxpedda This PR is ready for review, I think I handled all that was remaining from #24 :)

Just to note, given that the extra_cmd addition would have required changes in the github required checks anyway, I also took advantage of it to rename the checks into something less prone to change, which will require resetting the required checks this time but hopefully never thereafter.

[Fedeparma74]

Thank you @Ekleog for taking care of this!

[daxpedda]

Closing in favor of #28.