Add .sum method on Foldables using scala.Numeric

[kubukoz]

It would be good to have to enable summing NonEmptyLists like standard collections, similarly with other structures.

[kubukoz]

cc @hmemcpy

[hmemcpy]

Thanks! Yeah, I migrated some code from List to NonEmptyList and had to scratch my head for a second about .sum being an extension for SemigroupK. Replaced with .reduce and carried on, but it would be good to have .sum on a NEL that works as List.sum

[kubukoz]

Oh no, I didn't realise it was SemigroupK syntax... we'll need to find another name to avoid conflict, then.

[kubukoz]

I would suggest addAll.

And now that I think about it, we should have product too, but that's definitely overloaded as well from Semigroupal. So I'm thinking addAll / multiplyAll or sumAll / productAll.

[LukaJCB]

Deleted a comment that was spam

[johnynek]

I would really rather we not use scala.Numeric. I think we should instead be using Ring from algebra, which we have talked about merging into this repo as a separate subproject.

[GrigoriiBerezin]

Maybe we can use Semigroup in case the algebra is not merged into this repo yet?