Implement Drop for Directory in tsk.

[grrrrrrrrr]

Directory wraps a raw TSK_FS_DIR pointer obtained from
tsk_fs_dir_open or tsk_fs_dir_open_meta. In SleuthKit, directory
structures are dynamically allocated and must be freed with
tsk_fs_dir_close. Because Directory lacked a Drop implementation,
every opened directory leaked memory and underlying file handles.

This change implements Drop for Directory<'_> calling tsk_fs_dir_close
and relaxes iter_entries/file from &mut self to &self (the
underlying tsk_fs_dir_get takes const TSK_FS_DIR * and returns freshly
allocated files, so shared iteration is sound).

tsk_fs_dir_close also frees TSK_FS_DIR::fs_file, which is what
Directory::file hands back. Returning that File with the file system
lifetime 'a let it outlive the Directory, so once Directory has a
Drop a pattern like

let f = { let d = fs.open_dir(p)?; d.file() };

becomes a use-after-free. Directory::file now borrows self, tying the
returned File to the Directory it came from.

Note: File itself still has no Drop, so files from open_file and
iter_entries continue to leak TSK_FS_FILE. Fixing that needs an
owned/borrowed distinction (the walk_dir callback and Directory::file
must not close their File) and is left for a follow-up.