Fix for arguments defined prior to batching

Previously, the evaluation of the batch function did not search the frame that arguments were defined in.  This PR modifies batchman to also search `parent.frame(4)`, which contains arguments that are defined prior to the invocation of the batch function.

i.e., the following now works:
```R
predefined_var <- c(1, 2, 3)
some_batched_fn(predefined_var)