Function wasmtime_exn_field 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn wasmtime_exn_field(
    store: WasmtimeStoreContextMut<'_>,
    exn: &wasmtime_exn_t,
    index: usize,
    val_ret: &mut MaybeUninit<wasmtime_val_t>,
) -> Option<Box<wasmtime_error_t>>