blob: make the Blob struct more rust-like

[samlaf]

The Blob struct is written in a very dynamic-typed way, and doesn't make good use of rust's type system.

pub struct Blob {
    blob_data: Vec<u8>,
    is_padded: bool,
    length_after_padding: usize,
}

It would be cleaner to separate it into

#[derive(Clone, Debug, PartialEq, Eq)]
pub struct Blob {
    data: Vec<u8>,
}

#[derive(Clone, Debug, PartialEq, Eq)]
pub struct BlobPadded {
    data: Vec<u8>,
}

Benefits:

1. don't need extra "dynamic" state fields like is_padded and length_after_padding, you can just call blob.len() and blobPadded.len()
2. much cleaner API with explicit rust-like conversions between them (from/into/try_into/etc)
3. don't need to throw dynamic errors for things that could easily be caught at compile time, like to_polynomial which would only be implemented on BlobPadded.

    pub fn to_polynomial(&self, form: PolynomialFormat) -> Result<Polynomial, BlobError> {
        if !self.is_padded {
            Err(BlobError::NotPaddedError)
        } else {
            let fr_vec = helpers::to_fr_array(&self.blob_data);
            let poly = Polynomial::new(&fr_vec, self.length_after_padding, form)
                .map_err(|err| BlobError::GenericError(err.to_string()))?;
            Ok(poly)
        }
    }