chore(tvix/store): move NAR rendering logic into Renderer struct

This moves the logic rendering NARs to a struct using the
previously introduced, more granular BlobService, ChunkService and
DirectoryService.

Instead of passing them around to the helper functions, they're kept as
members of a struct.

Remove the async invocations in the nar_renderer tests, there's nothing
async in here.

Change-Id: Ic6d24aaad68a1fda46ce29f2cdb5f7b87f481d5c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8095
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/store/src/nar/mod.rs

@@ -0,0 +1,22 @@
+use data_encoding::BASE64;
+use thiserror::Error;
+
+mod renderer;
+
+pub use renderer::NARRenderer;
+
+/// Errors that can encounter while rendering NARs.
+#[derive(Debug, Error)]
+pub enum RenderError {
+    #[error("failure talking to a backing store client: {0}")]
+    StoreError(crate::Error),
+
+    #[error("unable to find directory {}, referred from {}", BASE64.encode(.0), .1)]
+    DirectoryNotFound(Vec<u8>, String),
+
+    #[error("unable to find blob {}, referred from {}", BASE64.encode(.0), .1)]
+    BlobNotFound(Vec<u8>, String),
+
+    #[error("failure using the NAR writer: {0}")]
+    NARWriterError(std::io::Error),
+}