Add `CheckedSum` and `CheckedProduct` traits#251

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wainwrightmark wants to merge 2 commits intorust-num:masterfrom wainwrightmark:master

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Commits on Nov 2, 2022

Added checked_product and checked_sum
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Commits on Nov 3, 2022

Made CheckedProduct and CheckedSum more consistent with Product and Sum traits.
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