Abbreviated time zones

[NilsIrl]

Support abbreviated time zones. e.g. BST, IST

[workingjubilee]

This isn't possible without an unambiguous mapping.

[quodlibetor]

Partially related to some of the use cases expressed in #54 , I think?

It won't be possible to construct a timezone from an abbreviated tz, but it should be possible to get the abbreviated name from a fully specified FixedTimespan

[pitdicker]

So creating a timezone from just an abbreviation is impossible, as mentioned before. Getting the abbreviation from a TzOffset is possible with the OffsetName trait.