feat: instances for literal and union types

[MartinHH]

• Arbitrary instances for literal types
• Arbitrary, Shrink and Cogen instances for union types

[MartinHH]

TODO: figure out how to properly attribute this.

According to official SO rules, this would need to be licensed under CC BY-SA 4.0 which probably not be a big problem as this will be used for testing only (and rather not redistributed), but will make the use of this library less pleasent than if it were licensed under something more inviting (like Apache 2.0).

I probably should try to bother Dmytro Mitin to get him to agree to something more reasonable than offical SO licensing.

[MartinHH]

So I now learned: according to the FSF, code snipetts of less than 15 lines are not legally significant + looking how I took the idea from SO and applied it to similar, but not identical code, I'd argue that this only took the soultion idea and did not copy any creative work.

Anyway, if you ever read this, Dmytro: I'm open for any suggestions how else to attribute you, I just did not want to bother you with this.