feat(tvix/store): eliminate generics in BlobStore

To construct various stores at runtime, we need to eliminate associated
types from the BlobService trait, and return Box<dyn …> instead of
specific types.

This also means we can't consume self in the close() method, so
everything we write to is put in an Option<>, and during the first close
we take from there.

Change-Id: Ia523b6ab2f2a5276f51cb5d17e81a5925bce69b6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8647
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/cli/src/main.rs

@@ -75,14 +75,14 @@ fn interpret(code: &str, path: Option<PathBuf>, args: &Args, explain: bool) -> b
     let directory_service = MemoryDirectoryService::default();
     let path_info_service = MemoryPathInfoService::default();
     let nar_calculation_service = tvix_store::nar::NonCachingNARCalculationService::new(
-        blob_service.clone(),
+        Box::new(blob_service.clone()),
         directory_service.clone(),
     );
 
     eval.io_handle = Box::new(tvix_io::TvixIO::new(
         known_paths.clone(),
         tvix_store::TvixStoreIO::new(
-            blob_service,
+            Box::new(blob_service),
             directory_service,
             path_info_service,
             nar_calculation_service,