Convenience and efficiency cleanups.

Added the methods Pile::size() and Pile::contains().
These methods, plus the other trivial Pile methods and the Card
comparison operators, are now defined inline in the class body,
as a micro-optimization.
Add "CheatBot" (avg score 24.1).

CheatBot does what its name implies: it uses global variables to
transfer information between the players. This means that every
player automatically knows (thanks to pleaseObserveBeforeMove())
the entire contents of his own hand at all times.

It's still very hard to come up with a strategy for getting the
best possible average score!