The cpio-archive package doesn't appear to support building NewC archives

[dralley]

There's a builder implementation for Odc archives, but not for NewC archives.

It would be nice if there was a builder implementation for the latter, or perhaps a shared builder which can write in a selectable format. I imagine much of the implementation can be shared.

[indygreg]

The cpio-archive crate is known to be in a woefully incomplete state: my initial goal of the crate was to facilitate reading Apple flat package (.pkg) files.

PRs to add missing features would be very much appreciated!