The "free" private method interferes with _CRTDBG_MAP_ALLOC

Visual Studio allows for the `_CRTDBG_MAP_ALLOC` macro to be defined, which replaces `malloc` and `free` with Windows' own macros for detecting memory leaks. As the preprocessor attempts to replace the `free` word in `void Config::free()`, this results in an error. What I personally did was rename the method to `void Config::do_free()`.