Support named enum in derive

[dannywillems]

See https://github.com/o1-labs/ocaml-gen/blob/main/ocaml-gen/derive/src/lib.rs#L214

For instance:

    #[derive(ocaml::IntoValue, ocaml::FromValue, ocaml_gen::Struct)]
    pub struct CamlRuntimeTableSpec {
        pub id: i32,
        pub len: usize
    }


    #[derive(ocaml::IntoValue, ocaml::FromValue, ocaml_gen::Enum)]
    pub enum CamlRuntimeTableCfg<CamlF> {
        Indexed(CamlRuntimeTableSpec),
        Custom {
            id: i32,
            first_column: Vec<CamlF>
        }
    }

will give:

    |
128 |     #[derive(ocaml::IntoValue, ocaml::FromValue, ocaml_gen::Enum)]
    |                                                  ^^^^^^^^^^^^^^^
    |
    = help: message: named types not implemented