Add a set_time method to DateTime

[pitdicker]

Example of where this would be useful: #1022.

Workarounds such as substracting the current time would not be correct.
You have to extract the current date as NaiveDate, add the desired time to get a NaiveDateTime, extract the original timezone, and construct a new DateTime.

Because it is definitely non-trivial to figure out, I propose to add this method.

[pitdicker]

Thinking about this a day later, it feels a bit like a hack. Or at least, incomplete.
When I think about DateTime, I see it as a combination of NaiveDate, NaiveTime, and something implementing TimeZone . It is already possible to swap out the timezone with with_timezone. This PR would add the ability to replace the time part. With a set_date method the set would be complete.

But I am starting to learn that with chrono you should have the mindset: when you want to do something non-obvious with DateTime, just drop down to its components such as NaiveDate and TimeZone, change what is needed, and recombine them.

Feel free to close this PR if you agree.

[djc]

Yeah, I think that makes more sense.