Consume a function's original implementation

[rodionovd]

For now, we don't pass a third argument original_ptr to the rd_route() call:

rd_route(rd_get_func_impl(targetMethod), rd_get_func_impl(replacement), nil)

so we lose an original implementation — that's a big problem, e.g. when we need to call it from new implementation (see issue #3).

Is there a way we can consume original_ptr in Swift?