Function get_fields 

fn get_fields(
    input: &DeriveInput,
    enum_variant: Option<&Variant>,
) -> Result<Fields>

Extract the Fields off a DeriveInput, or, in the enum case, off those of the enum_variant, when provided (e.g., for Zeroable).

We purposely allow not providing an enum_variant for cases where the caller wants to reject supporting enums (e.g., NoPadding).