Real dist release tarballs

We're currently relying on GitHub Releases's "download this tag" functionality for distribution tarbals. We should probably be creating real dist tarballs with a `make dist` target. To be a good FLOSS ecosystem citizen and avoid a hard dependency on GH publishing infrastructure. And for signing them, when we get around to that. ## References * [Ubuntu Upstream Guide](https://wiki.ubuntu.com/UpstreamGuide) * [its "tarball" section](https://wiki.ubuntu.com/UpstreamGuide#tarball)