Add K21 test example with BifunctorK type class

[joroKr21]

Followup to #221

[joroKr21]

@milessabin I'm not sure if we would actually want to add this to the library or just close it as an experiment

[milessabin]

If you have an actual use for it, then I don't see any objection to adding it, other than footprint.

This is all basically boilerplate ... could code gen be used? Then split an arbitrary number of kinds out into separate modules?

[joroKr21]

If you have an actual use for it, then I don't see any objection to adding it, other than footprint.

The only use-case I can think of is in cats-tagless for typelevel/cats-tagless#261 + typelevel/cats-tagless#29

This is all basically boilerplate ... could code gen be used?

I think that would be the only option yeah. I don't think macros or macro annotations can work currently.

Then split an arbitrary number of kinds out into separate modules?

Do you mean like ship the boilerplate generator? As like an sbt plugin? That would be an interesting idea.

[milessabin]

Do you mean like ship the boilerplate generator? As like an sbt plugin?

I didn't actually mean that, but it is an interesting idea!