refactor(tvix): move castore into tvix-castore crate

This splits the pure content-addressed layers from tvix-store into a
`castore` crate, and only leaves PathInfo related things, as well as the
CLI entrypoint in the tvix-store crate.

Notable changes:
 - `fixtures` and `utils` had to be moved out of the `test` cfg, so they
   can be imported from tvix-store.
 - Some ad-hoc fixtures in the test were moved to proper fixtures in the
   same step.
 - The protos are now created by a (more static) recipe in the protos/
   directory.

The (now two) golang targets are commented out, as it's not possible to
update them properly in the same CL. This will be done by a followup CL
once this is merged (and whitby deployed)

Bug: https://b.tvl.fyi/issues/301

Change-Id: I8d675d4bf1fb697eb7d479747c1b1e3635718107
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9370
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/castore/protos/rpc_directory.proto

@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: MIT
+// Copyright © 2022 The Tvix Authors
+syntax = "proto3";
+
+package tvix.castore.v1;
+
+import "tvix/castore/protos/castore.proto";
+
+option go_package = "code.tvl.fyi/tvix/castore/protos;castorev1";
+
+service DirectoryService {
+  // Get retrieves a stream of Directory messages, by using the lookup
+  // parameters in GetDirectoryRequest.
+  // Keep in mind multiple DirectoryNodes in different parts of the graph might
+  // have the same digest if they have the same underlying contents,
+  // so sending subsequent ones can be omitted.
+  rpc Get(GetDirectoryRequest) returns (stream Directory);
+
+  // Put uploads a graph of Directory messages.
+  // Individual Directory messages need to be send in an order walking up
+  // from the leaves to the root - a Directory message can only refer to
+  // Directory messages previously sent in the same stream.
+  // Keep in mind multiple DirectoryNodes in different parts of the graph might
+  // have the same digest if they have the same underlying contents,
+  // so sending subsequent ones can be omitted.
+  // We might add a separate method, allowing to send partial graphs at a later
+  // time, if requiring to send the full graph turns out to be a problem.
+  rpc Put(stream Directory) returns (PutDirectoryResponse);
+}
+
+message GetDirectoryRequest {
+  oneof by_what {
+      // The blake3 hash of the (root) Directory message, serialized in
+      // protobuf canonical form.
+      // Keep in mind this can be a subtree of another root.
+      bytes digest = 1;
+  }
+
+  // If set to true, recursively resolve all child Directory messages.
+  // Directory messages SHOULD be streamed in a recursive breadth-first walk,
+  // but other orders are also fine, as long as Directory messages are only
+  // sent after they are referred to from previously sent Directory messages.
+  bool recursive = 2;
+}
+
+message PutDirectoryResponse {
+  bytes root_digest = 1;
+}