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/tests/mod.rs

@@ -1,3 +1,3 @@
 mod directory_service;
-mod nar;
+mod nar_renderer;
 mod path_info_service;