refactor(tvix/castore): drop {Directory,File,Symlink}Node

Add a `SymlinkTarget` type to represent validated symlink targets.
With this, no invalid states are representable, so we can make `Node` be
just an enum of all three kind of types, and allow access to these
fields directly.

Change-Id: I20bdd480c8d5e64a827649f303c97023b7e390f2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12216
Reviewed-by: benjaminedwardwebb <benjaminedwardwebb@gmail.com>
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI

tvix/glue/src/builtins/derivation.rs

@@ -179,7 +179,7 @@ pub(crate) mod derivation_builtins {
     use nix_compat::nixhash::CAHash;
     use nix_compat::store_path::{build_ca_path, hash_placeholder};
     use sha2::Sha256;
-    use tvix_castore::{FileNode, Node};
+    use tvix_castore::Node;
     use tvix_eval::generators::Gen;
     use tvix_eval::{NixContext, NixContextElement, NixString};
     use tvix_store::proto::{NarInfo, PathInfo};
@@ -577,7 +577,11 @@ pub(crate) mod derivation_builtins {
                     })
                     .map_err(DerivationError::InvalidDerivation)?;
 
-            let root_node = Node::File(FileNode::new(blob_digest, blob_size, false));
+            let root_node = Node::File {
+                digest: blob_digest,
+                size: blob_size,
+                executable: false,
+            };
 
             // calculate the nar hash
             let (nar_size, nar_sha256) = state

tvix/glue/src/builtins/import.rs

@@ -3,7 +3,7 @@
 use crate::builtins::errors::ImportError;
 use std::path::Path;
 use tvix_castore::import::ingest_entries;
-use tvix_castore::{FileNode, Node};
+use tvix_castore::Node;
 use tvix_eval::{
     builtin_macros::builtins,
     generators::{self, GenCo},
@@ -213,7 +213,11 @@ mod import_builtins {
                     .tokio_handle
                     .block_on(async { blob_writer.close().await })?;
 
-                let root_node = Node::File(FileNode::new(blob_digest, blob_size, false));
+                let root_node = Node::File {
+                    digest: blob_digest,
+                    size: blob_size,
+                    executable: false,
+                };
 
                 let ca_hash = if recursive_ingestion {
                     let (_nar_size, nar_sha256) = state