write: support xz compression

[blalor]

It's more CPU-intensive, but results in much smaller ACIs.

[krobertson]

The reason I didn't add it when initially adding the compression support was because it'd add the dynamic library dependency. I don't like the compression handling that shells out to xz, but the dynamic linking then makes it required on the host.

One thing that may be worth trying though would be to make the pre-compiled binaries with each release be statically compiled...

[cgonyeo]

I'd be in favor of exploring statically compiled releases. It'd also help prevent other issues like when I accidentally build releases on NixOS: https://github.com/appc/acbuild/issues/133. There does appear to be a pure golang xz package, but it isn't stable so it's probably not worth it: https://github.com/ulikunitz/xz.

[jonboulle]

IIRC statically linking is probably not feasible because of resolver issues.

But in any case I don't understand how that relates exactly to the xz case? What does shelling out have to do with dynamic linking? unless you're suggesting baking a copy of xz into the releases or something..