Create a function to grab the arguments to layers or steps

Suppose

```r
r <- epi_recipe() %>%
  step_*() %>%
  ...
  step_blah(arg1, arg2)
```

We want something like `args_step(r, name = "blah")` that returns the arguments used in `step_blah()`. The same for `layer_*()` funs. 

Note that there is already `detect_step()` and `detect_layer()` that can be used.