zkvm: Protobuf based client/server IPC (risc0#814)

This introduces an API based on `protobuf` messages passed over a TCP
connection. The server is launched by the client as a sub-process. The
child process intended to be `r0vm` or other compatible binary that
executes for the duration of a request and then terminates. In this
scenario, the client is the one that binds to the TCP socket on port 0
to allocate a port. This port is passed to `r0vm --port <PORT>` and the
'server' then connects back to the listener. This ensures that ports are
unique for each caller in a race-free manner.

In this PR, the client/server messages are defined, while the core data
types (receipts, segments, etc) are specified as bytes. This means
there's an inner encoding that should be largely opaque to clients.
There are instances where clients will want to deserialize these bytes
(like a `Receipt`). Currently this uses `bincode` but the intention is
to move to `protobuf` to allow for a better versioning story. Versioning
will be fleshed out in a future PR.

Therefore there will be two levels of protobufs, one for the API, and
another for the core data types. This design follows best practices:
https://protobuf.dev/programming-guides/api/#use-different-messages

TODO:
* more tests especially around I/O
* pluggable implementation for lift & join

.vscode/settings.json

@@ -42,4 +42,4 @@
     "risc0/zkvm/methods/guest/Cargo.toml",
     "risc0/zkvm/methods/std/Cargo.toml"
   ]
-}
+}

risc0/cargo-risczero/src/commands/build_guest.rs

@@ -212,16 +212,16 @@ mod test {
     fn test_reproducible_methods_guest() {
         let tester = Tester::new("risc0/zkvm/methods/guest/Cargo.toml");
         tester.compare_image_id(
-            "risc0_zkvm_methods_guest/hello_commit",
-            "eb12f9b97d8759327f651afeb09ae9a5713e7dbc428284d453b8cf56e8dadd5a",
+            "risc0_zkvm_methods_guest/multi_test",
+            "851ad4ed2ade10e8f330737e36107f338f276e7036f992dd9da5010ff7fb6fea",
         );
         tester.compare_image_id(
-            "risc0_zkvm_methods_guest/multi_test",
-            "44750696d3d67caaaf9b1ca9441d1ffe5d5b9370d6945ce56b1972c10e42da59",
+            "risc0_zkvm_methods_guest/hello_commit",
+            "46c239f8f36b1e6dd239c0586b662fc1ea71cd593fe43a57903b974a69694b22",
         );
         tester.compare_image_id(
             "risc0_zkvm_methods_guest/slice_io",
-            "3f2ad1a2d500ab4ab927eebe241d872d3f598065b8987b182410cf01f350f74c",
+            "78d183c563f1ef6dc230c2dc504e29238ad284b175a5d1f6389b0e2649e0bd7b",
         );
     }
 }