Ch.3-6 Reworked bfs test so it doesn't take as long. Added convenience methods to EightPuzzleGen
Ch.3-6 Minor reformatting of EightPuzzleState
Ch.3-6 Added Frontier trait as using Traversable was problematic and buggy
Ch.3-6 Reduced visibility of Node to search package
Ch.3-6 Added GraphNodeExpanderTest, slightly modified GraphNodeExpander to add TreeNodeExpander as dependency, and fix buggy explored impl
Ch.3-6 Added TreeNodeExpanderTest
Updated copyright