Performance improvement for problem 82 by keeping tracking of next nodes to visit
Problem 82 performance increase x3 - don't both discarding nodes without unvisited neighbours
Rename cost fn to get-cost for analysis
Use some better parameter names
Problem 82: 10x speed increase by using reducer instead of sort to find cheapest node
Add profiling tags
P82: made complete? fn faster
Tidy up memoize
P82: store unvisited node data in tree