Streamline the release process

[exzombie]

qarv did not start on GitHub and the first releases were made by the make_release script, which made tarballs. GitHub allow releases to be made easily with git tags. CMake should be convinced to extract version from the latest tag as well; I like what the setuptools_scm plugin does for Python packages. The git tags use semantic versioning, so the API and ABI versions are in there, too.