Terminate loop early when possible. Add comments
Don't recurse into solve()
Refactor IO into ReadBoardFile() & ReadBoardSet()
Rename variables for clarity
go fmt
Print hardest puzzle
Improve error handling & misc refactor
Refactor ReadBoardFile() & ReadBoardSet() into BoardFile#Board & BoardSet#EachBoard
Fix path in README
Simplify Board type
Comments, minor reorganization & renaming
Update README