SafeNum should be written in terms of explicitly sized types like int32_t and int64_t

The current implementation of SafeNum makes unwarranted assumptions about the sizes of the integral types 'int' and 'long long int'. Since SafeNum is explicitly intended to model signed 32-bit and signed 64-bit integers, its interface and implementation should be re-written in terms of the appropriate fixed width integer types: int32_t and int64_t.