op_store: extract current Protobuf-based implementation to separate file

In order to allow building jj inside of Google, our Protobuf team
doesn't want to us to use a Google-unsupported implementation. Since
there is no supported implementation in Rust, we have to migrate off
of Protobufs. I'm starting with the operation store. This commit moves
the current implementation to a separate file so it can easily be
disabled by a Caargo feature.

lib/src/lib.rs

@@ -34,6 +34,7 @@ pub mod nightly_shims;
 pub mod op_heads_store;
 pub mod op_store;
 pub mod operation;
+mod proto_op_store;
 pub mod protos;
 pub mod refs;
 pub mod repo;