Option to encode single-constructor types as tagged sums

[Lysxia]

As suggested in #206 and possibly #473

[bergmark]

Github showed the already merged PR's in the history so i force pushed a rebase.

[bergmark]

Thanks again, This looks good as well!

Unavoidably this is a breaking change due to the Options constructor being exposed. I'll file an issue for un-exposing in the next major along with this to avoid this in the future.

I'll cut a release with the other PR's tomorrow and then merge this.

[bergmark]

Looks like i also confused myself and travis by doing "git push origin Lysxia/feature-474" so don't mind the test failure!

[bergmark]

I noticed that the new field needs to be added to the Show Options instance. I added a regression test so rebasing this PR should break. This should be rebased on top of #526, but let's save some effort and wait for that to get merged!

[bergmark]

#526 was merged so this can be rebased.

[bergmark]

Same here, I didn't get notified of the recent changes.

Thanks a lot for this!

[Lysxia]

Okay! You're welcome!

Actually this patch may lead to some serious regressions because I've introduced an actual loop in the generic implementation. We can probably avoid it by tracking the new option at the type level, as a parameter of the GToJSON type class, or by splitting that class in some way.

[Lysxia]

This is now just two simple overlapping instances instead of the previous recursive mess.

Ready to merge.

[bergmark]

Great stuff!