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feat(algebra/big_operators/finprod): assorted lemmas
These lemmas come from an old version of #8903. I don't need them anymore, so feel free to reject or drop some of the lemmas before merging.
Until someone picks this up, I will remove the awaiting-review
label.
I don't think this needs much improvement; but otoh, it's not clear that we need this statements. So until the need is there, I don't think we should merge this. Agreed?