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/Cargo.toml

@@ -0,0 +1,42 @@
+[package]
+name = "tvix-castore"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+anyhow = "1.0.68"
+async-stream = "0.3.5"
+blake3 = { version = "1.3.1", features = ["rayon", "std"] }
+bytes = "1.4.0"
+data-encoding = "2.3.3"
+futures = "0.3.28"
+lazy_static = "1.4.0"
+pin-project-lite = "0.2.13"
+prost = "0.11.2"
+sled = { version = "0.34.7", features = ["compression"] }
+thiserror = "1.0.38"
+tokio-stream = { version = "0.1.14", features = ["fs", "net"] }
+tokio-util = { version = "0.7.8", features = ["io", "io-util"] }
+tokio = { version = "1.28.0", features = ["fs", "net", "rt-multi-thread", "signal"] }
+tonic = "0.8.2"
+tower = "0.4.13"
+tracing = "0.1.37"
+url = "2.4.0"
+walkdir = "2.4.0"
+
+[dependencies.tonic-reflection]
+optional = true
+version = "0.5.0"
+
+[build-dependencies]
+prost-build = "0.11.2"
+tonic-build = "0.8.2"
+
+[dev-dependencies]
+test-case = "2.2.2"
+tempfile = "3.3.0"
+tonic-mock = { git = "https://github.com/brainrake/tonic-mock", branch = "bump-dependencies" }
+
+[features]
+default = ["reflection"]
+reflection = ["tonic-reflection"]