Replace `ZeroVec` with Newtype Wrapper for Human-Readable JSON and `BigInt` Conversion in `ConstantValue`

[younies]

Instead of using a plain ZeroVec for both the numerator and denominator in the ConstantValue, consider creating a newtype wrapper around ZeroVec. This wrapper could implement human-readable JSON serialization. Additionally, it should include functions for converting to and from BigInt.