feat(tvix/store): add write_nar function

This adds a function that consumes a [proto::node::Node] pointing to the
root of a (store) path, and writes the contents in NAR serialization to
the passed [std::io::Write].

We need this in various places:

 - tvix-store's calculate_nar() RPC method needs to render a NAR stream
   to get the nar hash, which is necessary to give things imported in
   the store a "NAR-based" store path.

 - communication with (remote) Nix (via daemon protocol) needs a NAR
   representation.

 - Things like nar-bridge, exposing a NAR/NARInfo HTTP interface need a
   NAR representation.

Change-Id: I7fb2e0bf01814a1c09094c0e35394d9d6b3e43b6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7956
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/store/src/lib.rs

@@ -1,8 +1,11 @@
+pub mod client;
 pub mod proto;
 
 pub mod dummy_blob_service;
 pub mod sled_directory_service;
 pub mod sled_path_info_service;
 
+mod nar;
+
 #[cfg(test)]
 mod tests;