Clean up repr handling in zerocopy-derive

[djkoloski]

It might be cleaner and more reliable to do something like:

enum DiscriminantRepr {
    C,
    Primitive(...),
}

impl DiscriminantRepr {
    fn try_extract_from_reprs(reprs: &[EnumRepr]) -> Result<DiscriminantRepr, ...> { ... }
}

Then we could make derive_is_bit_valid more structural:

pub(crate) fn derive_is_bit_valid(
    reprs: &[EnumRepr],
    ident: &Ident,
    generics: &Generics,
    data: &DataEnum,
) -> TokenStream {
    let idents = RawIdents::for_enum(ident);

    match DiscriminantRepr::try_extract_from_reprs(reprs) {
        Ok(DiscriminantRepr::C) => derive_is_bit_valid_c(data, generics, &idents),
        Ok(DiscriminantRepr::Primitive(prim)) => derive_is_bit_valid_primitive(data, generics, prim, &idents),
        Err(...) => ...,
    }
}

Originally posted by @joshlf in #879 (comment)