#4 Map API

[Ted Turocy]

The Gambit::Map API was designed in the mid-1990s before exceptions were part of the C++ standard. As a result, the class uses a kludgey "default" which is returned when a requested key is not defined. In addition, some function names are not consistent with more recent developments. The API should be modernized to use exceptions and operator overloading, and work towards consistency with other comparable classes in, e.g., BOOST.