allow users to get ZstdLayerWriter directly

[tofay]

... rather than wrapping in a tar builder. Apologies for missing this in #24

In rpmoci I'm wanting to give the user a choice of gzip or zstd compression. I want to create an enum like

enum LayerWriter<'a> {
    Gzip(ocidir::GzipLayerWriter<'a>),
    Zstd(ocidir::ZstdLayerWriter<'a>),
}

impl<'a> Write for LayerWriter<'a> {
    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
        match self {
            Self::Gzip(writer) => writer.write(buf),
            Self::Zstd(writer) => writer.write(buf),
        }
    }

    fn flush(&mut self) -> std::io::Result<()> {
        match self {
            Self::Gzip(writer) => writer.flush(),
            Self::Zstd(writer) => writer.flush(),
        }
    }
}

I'll then create a tar::Builder<LayerWriter<'a>>. But I can't currently create an ZstdLayerWriter.

[ariel-miculas]

Suggested change

	let dir = Dir::open_ambient_dir(env::temp_dir(), ocidir::cap_std::ambient_authority()).unwrap();
	let dir = Dir::open_ambient_dir(env::temp_dir(), ocidir::cap_std::ambient_authority()).unwrap();
	let tmpdir = "zstd-example";
	dir.create_dir_all(tmpdir).unwrap();
	let dir = dir.open_dir(tmpdir).unwrap();

I suggest creating a separate directory in /tmp for the oci directory, otherwise we end up with /tmp/index.json and /tmp/blobs/sha256.