rust-inner-revlog: move to `RwLock` instead of `RefCell` We need to make `InnerRevlog` `Sync`, since we want to iterate on and read it in parallel. An uncontended `RwLock` has no measurable overhead over a `RefCell` in our contexts (to be verified in benchmarks, but I'm pretty sure). This change assumes that writes/reads to/from the uncompressed chunk cache can fail if contended, since it's just a cache, but the rest of operations (which are on FileHandles) should wait for the lock and bubble up the panic in case of lock poisoning. If a program tries to write from multiple threads to a revlog, it had better have good reasons, so I'm not too worried.