Add direct constructors for OffsetDateTime.

[jcdyer]

Fixes #640

This PR adds new constructors to OffsetDateTime:

• OffsetDateTime::new_in_offset(date: Date, time: Time, offset: UtcOffset) -> Self
• OffsetDateTime::new_utc(date: Date, time: Time) -> Self

Rationale for the PR can be found on the linked issue.

[jcdyer]

Can someone help me interpret the output of the coverage build, and understand what actions I should take? To my eye, it looks like a failed snapshot in a part of the codebase I didn't touch. Let me know if I'm missing something, and how I can resolve it.

[jhpratt]

Don't worry about code coverage; the output is what I expect. It's currently slightly broken due to a bug in the compiler that I need to open an issue for. However, tests for these methods would be appreciated.

For the API itself, I would prefer not to have now_local. It would be quite error prone given that UTC offsets change throughout the year in most of the western world. new_utc is good as-is, though I'd like to see new renamed to new_in_offset for clarity.

[jcdyer]

I tend to prefer new over new_in_offset, as I think it is clear enough from the required arguments that it includes the offset, and this is the most natural constructor for an OffsetDateTime. But that's a minor quibble, and I'm happy to do it the way you think makes the most sense.

Changes made as requested, tests added, and ready for a new review.

[jhpratt]

I just realized that this is only implemented in offset_date_time.rs and not in date_time.rs. Given that I'm reworking some of the internals in the latter at the moment, I'll handle that.

This PR looks good. Merging!