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/src/digests.rs

@@ -0,0 +1,72 @@
+use bytes::Bytes;
+use data_encoding::BASE64;
+use thiserror::Error;
+
+#[derive(PartialEq, Eq, Hash, Debug)]
+pub struct B3Digest(Bytes);
+
+// TODO: allow converting these errors to crate::Error
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("invalid digest length: {0}")]
+    InvalidDigestLen(usize),
+}
+
+impl B3Digest {
+    // returns a copy of the inner [Vec<u8>].
+    pub fn to_vec(&self) -> Vec<u8> {
+        self.0.to_vec()
+    }
+}
+
+impl From<B3Digest> for bytes::Bytes {
+    fn from(val: B3Digest) -> Self {
+        val.0
+    }
+}
+
+impl TryFrom<Vec<u8>> for B3Digest {
+    type Error = Error;
+
+    // constructs a [B3Digest] from a [Vec<u8>].
+    // Returns an error if the digest has the wrong length.
+    fn try_from(value: Vec<u8>) -> Result<Self, Self::Error> {
+        if value.len() != 32 {
+            Err(Error::InvalidDigestLen(value.len()))
+        } else {
+            Ok(Self(value.into()))
+        }
+    }
+}
+
+impl TryFrom<bytes::Bytes> for B3Digest {
+    type Error = Error;
+
+    // constructs a [B3Digest] from a [bytes::Bytes].
+    // Returns an error if the digest has the wrong length.
+    fn try_from(value: bytes::Bytes) -> Result<Self, Self::Error> {
+        if value.len() != 32 {
+            Err(Error::InvalidDigestLen(value.len()))
+        } else {
+            Ok(Self(value))
+        }
+    }
+}
+
+impl From<&[u8; 32]> for B3Digest {
+    fn from(value: &[u8; 32]) -> Self {
+        Self(value.to_vec().into())
+    }
+}
+
+impl Clone for B3Digest {
+    fn clone(&self) -> Self {
+        Self(self.0.to_owned())
+    }
+}
+
+impl std::fmt::Display for B3Digest {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "b3:{}", BASE64.encode(&self.0))
+    }
+}