Almost all of the internal code in the mpsc module makes use of &mut self, which isn't valid because it's all explicitly designed to work across multiple threads, meaning the guarantees of &mut self don't hold. This entire implementation dates back to Fall 2013, which is long before the multithreading of &self vs &mut self was even decided!

We should update these implementation details to basically never use &mut self but instead use UnsafeCell where appropriate.