0.5: Add a LocalOffset type

[pitdicker]

Currently the type returned for DateTimes with a Local time zone is a FixedOffset. This makes it impossible to add information to the DateTime such as the time zone name, abbreviation, or DST status.

For 0.5 we should instead return a LocalOffset type. That will allow us to expose more information in the future.

[djc]

Appreciate all the research into this!

At the same time, I'm wary of adding substantial complexity to chrono on account of a tiny majority of use cases. Correctness and precision are not exactly the same thing, and it feels to me like you're chasing a level of precision here the requirement for which might be extremely rare, and, as such, not a great fit for a general purpose library such as chrono.

The only blocker here for 0.5 seems to be that we have a separate type LocalOffset which can expose more information. We could also seal the Offset trait so that we can more easily add functions to it later.

[pitdicker]

Sorry, replied in the wrong issue. Yes, I would only like to see a LocalOffset with nothing else for 0.5. The rest would not be a breaking change.