Fixed search bug reported by Matthew Yurka, and others:

 * graph_search missed deduping nodes in the frontier
 * astar_search didn't use 3rd-edition pseudocode
 * InstrumentedProblem didn't forward the path_cost method,
   so all the doctest results on instrumented cost-sensitive
   problems were wrong

Also filled in breadth_first_search and per-node memoization 
of h (to fulfill the doc comment saying that was done).

There's a remaining problem that sometimes trivial refactorings
change the amount of search that is done, slightly, which should
not happen since the code is supposed to be deterministic.