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

Agent.__init__ now takes a program parameter. A few more nonlocal-state-variable problems found and fixed. One agent program rewritten as a callable instance since it uses both state variables and abstract methods.

Moved agent-program definition back into the constructors to get rid of all the copying between parameters, instance vars, and nonlocals. Fixed nonlocal state-var bug in ReflexAgentWithState.

Some genetic_search() fixes: states were lists but had .mutate() called on them; pmut was probability of *not* mutating; initial population nonrandom; bad fitness function.

of the NQueensProblem -- it had been ignoring conflicts with the
last queen previously added.