maybe-union-with add the function and comments

[perkee]

WIP needs

• tests (unless verified examples are ok)
• benchmarks of the various approaches
  • is casing on a 2-tuple slow?
  • Is nesting a case better
• is the name OK? Should most-everything in that comment move into this PR description?

[gampleman]

Hi @perkee, thanks for this PR.

Could you show/explain some real world code where this function would have been useful and perhaps how some of the alternatives were inferior?

I have to confess that I have personally never thought of needing such a function, but maybe it comes up all the time and I just haven’t realised. So seeing some real world use cases would help us evaluate this. Thank you.