message: Derive: allow negative enum discriminants
diff: @@ -68,7 +68,7 @@ fn h5type_impl_enum(names: Vec<&Ident>, values: Vec<&ConstExpr>,
  members: vec![#(
  ::hdf5_types::EnumMember {
  name: stringify!(#names).to_owned(),
  - value: (#values) as u64,
  + value: (#values) as i64 as u64,
  }
  ),*],
  }
  