refactor(tvix/castore): add into_nodes(), implement consuming proto conv

Provide a into_nodes() function on a Directory, which consumes self and
returns owned PathComponent and Node.

Use it to provide a proper conversion from Directory to the proto
variant that doesn't clone.

There's no need for the one taking only &Directory, we don't use it
anywhere, and once someone needs that they might as well clone Directory
before converting it.

Update all other users of the `.nodes()` function to use `.into_nodes()`
where applicable, and avoid some more cloning there.

Change-Id: Id4577b9eb173c012e225337458898d3937112bcb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12218
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/castore/src/nodes/directory.rs

@@ -50,6 +50,12 @@ impl Directory {
         self.nodes.iter()
     }
 
+    /// Dissolves a Directory into its individual names and nodes.
+    /// The elements are sorted by their names.
+    pub fn into_nodes(self) -> impl Iterator<Item = (PathComponent, Node)> + Send + Sync {
+        self.nodes.into_iter()
+    }
+
     /// Adds the specified [Node] to the [Directory] with a given name.
     ///
     /// Inserting an element that already exists with the same name in the directory will yield an