Add TODO.md
Create basic graph classes
Add some print-object methods and move to new files
Initial hash-set code
Trivial tests pass
Semi-finalise API
Implement hs-delete
Implement hs-union, hs-intersection and hs-map
Implement hs-equal
Implement hs-copy, and hs-difference
Implement hs-symmetric-difference
Implement hs-to-list and fix hs-pretty-print so that it handle nested hash-sets
Clean up linked lists
Remove dir hash-set