sketchlib v0.2.0

This is the first stable version. We will attempt to keep the file formats consistent going forwards.

This includes inverted index support, and various bug fixes (notably for knn distances).

What's Changed

• Inverted index by @johnlees in #41
• Add support for inverted index queries by @johnlees in #42

Full Changelog: v0.1.3...v0.2.0