refactor(tvix/store): use composition in tvix_store crate

Change-Id: Ie6290b296baba2b987f1a61c9bb4c78549ac11f1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11983
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: yuka <yuka@yuka.dev>
Tested-by: BuildkiteCI

tvix/castore/src/composition.rs

@@ -454,4 +454,11 @@ impl Composition {
         *entry = Box::new(new_val);
         ret
     }
+
+    pub fn context(&self) -> CompositionContext {
+        CompositionContext {
+            stack: vec![],
+            composition: Some(self),
+        }
+    }
 }