add wrapping/checked/saturating assist
This addresses #13452
I'm not sure about the structure of the code. I'm not sure if it needs to be 3 separate assists, and if that means it needs to be in 3 separate files as well.
Most of the logic is in `util.rs`, which feels funny to me, but there seems to be a pattern of 1 assist per file, and this seems better than duplicating the logic.
Let me know if anything needs changes 😁
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