Class CountValue

object --+    
         |    
   Counter --+
             |
            CountValue

A counter class for simple, atomic memoized values.

A CountValue object should be instantiated in a class for each of
the class's methods that memoizes its return value by simply storing
the return value in its _memo dictionary.

We expect that the metaclass initialization will fill in the
.underlying_method attribute with the method that we're wrapping.
We then call the underlying_method method after counting whether
its memoized value has already been set (a hit) or not (a miss).