src/backup/catalog_blob.rs: moved catalog impl. from pxar

And avoid loading catalog into memory.

src/backup.rs

@@ -146,6 +146,9 @@ pub use data_blob_reader::*;
 mod data_blob_writer;
 pub use data_blob_writer::*;
 
+mod catalog_blob;
+pub use catalog_blob::*;
+
 mod chunk_stream;
 pub use chunk_stream::*;