Function get_external 

pub unsafe extern "C" fn get_external(
    context: *mut c_context,
    value: *mut value,
) -> *mut ExternalValue

@brief Get external reference @ingroup value_extract @param[out] context Optional, stores error information @param[in] value Nix value to inspect @return reference valid while value is valid. Call nix_gc_incref() if you need it to live longer, then only in that case call nix_gc_decref() when done. NULL in case of error