ZCdpBudget::new() should be fallible, error on epsilon=0

[divergentdave]

Most DP mechanisms are only well-defined for $\varepsilon > 0$. We disallow negative values by using Ratio<BigUint>, but we don't yet reject zero. Currently, trying to use an epsilon of zero results in a panic due to division by zero in create_distribution(). We should make the budget constructor fallible, and check for this.