/// `cleanup` functions (one for each data structure), the `init` function for
/// `TF_Filesystem` and the `new_*` methods of `TF_FilesystemOps` to initialize
/// the 3 types of files. In all other cases, there is no need to modify the
/// address of the opaque data pointer, hence the wrapper pointer is marked
/// `const`.
///
/// For consistency, the arguments on all these functions follow the same