Fixing Bundler's dependency resolution algorithm and making it 2x faster

A month ago we had a report of some strange behaviour from Dependabot: on some projects, we were creating a “Dependabot can’t resolve your Ruby dependency files” issue, only to immediately close it. Hunting down that bug took me right to the core of Bundler’s dependency resolution logic, and I spent three weeks working to improve it.Happily, the result is a 2x speedup for Bundler’s resolution logic and a bunch of fixes that mean it now has no known bugs.