program database storage refactor

[tlh24]

Per notes from Tues night:

1. Replace Vector with a fixed Array
2. Keep track of unused slots in the array with a list
3. Remove unused nodes periodically, without corrupting the rest of the graph.
   Otherwise we end up running out of memory for the working programs!
   (Certainly can make things a good bit larger, but still..)

working on new branch vec2arr for this.