Function vid_disperse_conversion 

fn vid_disperse_conversion(
    vid_disperse: VidDisperse<Advz<Bn254, Sha256>>,
) -> VidDisperse<ADVZScheme>

Convert a [VidDisperse<Advz>] to a [VidDisperse<VidSchemeType>].

Foreign type rules prevent us from doing:

• impl From<VidDisperse<VidSchemeType>> for VidDisperse<Advz>
• impl VidDisperse<VidSchemeType> {...}

and similarly for Statement. Thus, we accomplish type conversion via functions.