Revise NSDictionary and NSMutableDictionary

NSDIctionary and NSMutableDictionary Value methods all take a Ptr argument, and return a Ptr.  My experience with other code suggests that this is the right interface for now.  All other approaches involve endless typecasting or the use of lookup table that requires maintenance.  In most cases, one knows the type of the value returned for a given key, and thus one can simply pass the Ptr returned for a key to the correct constructor.