In the iterator function which is the low-level API, don't depend on
`std.fs.MAX_PATH_BYTES` because this is not defined on all operating
systems, such as freestanding.

However in such environments it still makes sense to be able to extract
from a tar file.

An even more flexible solution would be to accept the buffers as
arguments to iterator() which I think is a good idea, but for now let's
just set the same upper limmit across all operating systems.