add IndexFile trait

We want to be able to iterate through digests of index
files, but without always having to distinguish between
dynamic and fixed types, so add a trait we can use as a
trait object.

Unfortunately the iterator needs to yield copies as
iterators cannot yield values with life times when
represented as trait objects (Box<dyn Iterator<Item = ?>>)

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

src/backup.rs

@@ -20,6 +20,9 @@ pub use chunker::*;
 mod chunk_store;
 pub use chunk_store::*;
 
+mod index;
+pub use index::*;
+
 mod fixed_index;
 pub use fixed_index::*;