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/castore/src/proto/tests/directory.rs

@@ -301,7 +301,7 @@ fn validate_sorting() {
         };
         match crate::Directory::try_from(d).expect_err("must fail") {
             DirectoryError::DuplicateName(s) => {
-                assert_eq!(s, b"a");
+                assert_eq!(s.as_ref(), b"a");
             }
             _ => panic!("unexpected error"),
         }