Enable ffill for datetimes

[max-sixty]

Notes inline. Would fix #4587

[max-sixty]

Actually this is tougher than I realized. I'll close given this approach won't work. Open to ideas if anyone has a way of doing it...

[dcherian]

Use .view(np.int64) instead?

[max-sixty]

Use .view(np.int64) instead?

But unfortunately np.int64 isn't nullable, so filling no longer makes sense...

[dcherian]

NaT is np.iinfo.min() IIRC, so there's a path

>>> np.datetime64('nat').view(np.int64)
-9223372036854775808

[max-sixty]

Good point!

We would need to special-case that. Maybe a new numbagg routine... (though tbh I'm not sure how often folks are forward-filling datetimes. But would def accept a PR if someone wants to add...)