feat(tvix/store): implement iteration over Directory nodes

This provides a `Directory.nodes()` function, returning an iterator over
all three node types in an ordered fashion.

Change-Id: Ib98696c03a9db8b6c613d6e2bf5587c1ae35133f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7955
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/store/src/tests/mod.rs

@@ -1,4 +1,5 @@
 mod directory;
+mod directory_nodes_iterator;
 mod directory_service;
 mod path_info_service;
 mod pathinfo;