Make std.tar look better in docs. Remove from public interface what is
not necessary. Add comment to the public methods. Add doctest as usage
examples for iterator and pipeToFileSystem.
Like in issue #18089, this tar contains, same file name in two case
sensitive name version. Unpack should fail on case insensitive file
systems and succeed on case sensitive.
$ tar tvf 18089.tar
18089/
18089/alacritty/
18089/alacritty/darkermatrix.yml
18089/alacritty/Darkermatrix.yml
Running fuzzing tar test with [zig std lib
fuzzing](https://github.com/squeek502/zig-std-lib-fuzzing) reached and
assert in tar implementation. Assert (in std lib) should not be
reachable by external input, so I'm fixing this to return error.