{vanilla} Clean up custom print function

[asmaloney]

The code for the custom print function is found in framework/vanilla_actr/vanilla_print.lisp.

Right now the custom printing of a buffer print goal produces something like:

  goal: GOAL-CHUNK0
    NUM1  3
    NUM2  1
    COUNT  EMPTY
    SUM  EMPTY

What I want is more like this:

  goal: add( count=empty, num1=3, num2=1, sum=empty )

1. I can't figure out how to get the "isa" slot from the chunk (GOAL-CHUNK0 -> add in this case). (Even though they are optional, they do exist in our code.)

2. Displaying each of the slots happens in printed-chunk, but I don't understand Lisp enough to pick it apart & format it the way I want to.

If you know Lisp, please feel free to fix this and submit a PR!