Initial standalone version

This version updates the original[1] repo's v1.0.1.

New features:
* AddFS method for writing a whole fs.FS to the archive.
* Preliminary support for fs.ReadLinkFS.
* Supports symbolic links.

Breaking changes:
* Remove linkname from FileInfoHeader as it was not really used for
  anything.

[1]: https://github.com/cavaliergopher/cpio