Feature: linkage fixtures to track changes

This is an interesting idea proposed by @ashleygwilliams.

We track linkage data in metadata stored after the build, which does give a historical timeline of linkage changes over time. What if we stored and used linkage data as a kind of snapshot like the [insta](https://crates.io/crates/insta) snapshots we use for our own tests? CI builds could then flag if linkage has changed from the snapshots, providing important information to devs. It could even be used to cause builds to fail if desired in order to avoid possibly broken binaries.