Expose macro_export in guest-rust-macro

[AmitPr]

This PR exposes the option to pass export_macro as an argument, which allows one to do something like:

wit_bindgen_guest_rust::generate!({
    default: "./wit/http_service.wit",
    name: "http_component",
    macro_export,
});

For context, setting the macro_export option to true adds a #[macro_export] to the generated export_{module_name} macro.

[alexcrichton]

Thanks!