refactor(tvix/castore): add PathComponent type for checked components

This encodes a verified component on the type level. Internally, it
contains a bytes::Bytes.

The castore Path/PathBuf component() and file_name() methods now
return this type, the old ones returning bytes were renamed to
component_bytes() and component_file_name() respectively.

We can drop the directory_reject_invalid_name test - it's not possible
anymore to pass an invalid name to Directories::add.
Invalid names in the Directory proto are still being tested to be
rejected in the validate_invalid_names tests.

Change-Id: Ide4d16415dfd50b7e2d7e0c36d42a3bbeeb9b6c5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12217
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI

tvix/nar-bridge/src/nar.rs

@@ -57,7 +57,7 @@ pub async fn get(
         StatusCode::BAD_REQUEST
     })?;
 
-    if !root_name.is_empty() {
+    if !root_name.as_ref().is_empty() {
         warn!("root node has name, which it shouldn't");
         return Err(StatusCode::BAD_REQUEST);
     }