New approach: Toolpath is a python program

Another way of saying this is that the VM provides a scripting interface, whose native language is python.
The following example serves as our current specification. We'll get a bit more formal about it as we go.

Auto-generated toolpath instructions for routing out a square
Manually created toolpath instructions for routing out the same square (easier to read)
A sample VM definition on which the "square" toolpath runs

What we get when we run the "square" toolpath on the sample VM implementation:

sl@marmot:~/make2$ python square.py 
If I were really a machine, I would wait here until the user pressed my button.
Setting speed in z axis to 2
Setting speed in xy axis to 10
Moving to position 0.1 in z axis.
Moving to position X= 0 , Y= 0
Setting speed in bit axis to 3600
Turning cutting bit ON
Moving to position -0.01 in z axis.
Moving to position X= 1 , Y= 0
Moving to position X= 1 , Y= 1
Moving to position X= 0 , Y= 1
Moving to position X= 0 , Y= 0
Moving to position 0.1 in z axis.
Turning cutting bit OFF
Moving to position X= -1 , Y= -1 , Z= 0.2