A question would be do we want to use GMP or are 64 bit ints enough? I'm inclined to say ints are ok of they are wide.
Sorry to jump to such a basic question. I sort of think it helps frame a discussion about what people really want.
I mostly needed addition, comparison and decrement of values that might be as large as a file or a rim image at worst.
So for me an add function that took signed numbers would be enough if it came with < > = since we already have "or" and "and". e.g. X:=$(add $(X),-1) gives you decrement.
If one is minimal then there is less chance of creating a carbuncle.