Add Into<BigInt> bound to FieldElementWithInteger::Integer

[divergentdave]

We could clean up our TypeWithNoise implementations if we added a bound to the Integer associated type that provided conversions to and from BigInt. This is needed because we have to take the modulus of the noise to be added in the BigInt domain, before projecting into the field. (This operation is primarily necessary to wrap around negative noise values, but it is moreover necessary so that we correctly implement the DP mechanism on integers, and then project into the field as a post-processing step, preserving the DP guarantees) This trait change should wait for our next batch of breaking changes.