Support nested directory reads for zip file creation

From #38, when the zip file `ReadDir` instance comes along a directory, it currently panics due to a `todo!`. We should fix this sooner than later.

https://crates.io/crates/walkdir is pretty much the standard way of recursively walking a directory.

Start at the zip-file creation path where the ReadDir instance encounters a directory and hits the todo!; inspect how that path currently reads entries. The issue suggests the walkdir crate for recursive traversal. Done means nested directory reads no longer panic during zip creation.

Written by the indexing model from the issue text.