* 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.

Don't compare strings with 'is not'. (Broke things in pypy.) Also show Struct members in sorted order so doctests work in pypy.

Replaced Learner with a functional interface. This is a pretty big change you might especially want to review.

Fixed DataSet doc comments. Added rep-invariant check. DecisionTreeLearner now breaks ties on leaves randomly. NaiveBayesLearner now collects stats only on input fields. Minor cleanup.