Implement Add, Sub, CheckedAdd, CheckedSub for Uints

[ValarDragon]

Summary

We should add implementations of Add, Sub, CheckedSub and CheckedAdd for Uints. These are common functions one needs when working with Uints. Currently there only exists AddMany

CheckedAdd and CheckedSub should be unsatisfiable if there is an overflow. I think the simplest checked arithmetic implementation would be to just compute the final carry bit and ensure its 0

(The checked and non-checked methods can be implemented in separate PRs!)

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