More robust handling of public parameters

[larskuhtz]

I think the following code snipped it problematic, since it rules out programs with public parameters that are shorter than 32 bytes.

    let public_inputs: String = if bit_length > 256 {
        // The user has provided the committed values rather than the digest!
        // Let's reproduce the digest using the committed values

It think, the best solution would be to provide a separate API function for the (probably common) use case when the actual public parameters are provided.