This adds a step and a rule for building the error index as part of rustbuild.
Add a script to get run which verifies that `href` links in documents are correct. We're always getting a steady stream of "fix a broken link" PRs and issue reports, and we should probably just nip them all in the bud.
src/tools
We've actually got quite a few tools that are compiled as part of our build, let's start housing them all in a `tools` directory.
