Consume a function's original implementation

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

``` swift
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?
