0.4.23 release planning

[djc]

Requested in #849. I'd like to wait until #844 has been merged (should be soon).

• Deprecate methods that have an _opt() alternative (0.4.x: deprecate methods that have an _opt() alternative #827)
• Addition and subtraction with the Days type (add/sub Days #784)
• Add NaiveDateTime::from_timestamp_millis(_opt) (NaiveDateTime::from_timestamp_millis(_opt) #818, thanks to @Pscheidl -- backported in Port from_timestamp_millis() (#818) to 0.4.x #823)
• Add optional support for the arbitrary::Arbitrary trait (Arbitrary 04x #849, thanks to @greyblake and @asayers)
• Support tzdb location on AIX (Support AIX's TZDB location #826)
• Fix warnings in documentation (add backticks in docs #847)

[djc]

I think at this point we're also going to want to include #851 and #853. I'm on holiday this week so might be slower to get through reviews for a bit.

[djc]

It's done. https://github.com/chronotope/chrono/releases/tag/v0.4.23