gigasecond: Completely rewrite the exercise or deprecate it.

[rbasso]

The way it is now, gigasecond has some problems:

• It is too easy and boring.
• All answers are almost the same: fromDay = addUTCTime 1e9
• Does not match exercism/x-common/gigasecond.json.

Seems that, at least in other tracks, it can be interesting: #192 (comment)

In the Go track gigasecond is a fantastic exercise. People tackle it in so many different ways, and there are myriad opportunities to talk about idioms in the language and certain language features.

I think that if it can be turned into a more interesting exercise, then that's worth doing. If not, just deprecate it.

Edit: This exercise was changed in branch hspec-fail-fast. Please use that branch for any PR related to it or wait until it's merged in master.

[kytrinyx]

I'm totally OK with deprecating it if it's too boring and we can't come up with a more interesting variant.

[rbasso]

Until now, there is no proposal about how to make gigasecond an interesting exercise, so I think it's best to deprecate it now. If anyone finds a way to make it a good exercise in Haskell, we can easily undeprecate it.