Decide how to handle reedmuller-c

Summary

We have a modified copy of the reedmuller-c library in the repo. Currently we treat it as an external library: for instance, we use the same coding style as upstream.

Since upstream is dead and we have local modifications, we could instead acknowledge the fork and integrate the lib with our tooling (which currently means clang-formatting it).

Let's vote: 🦅 for current status (external lib) and 🏝 for integration.