- Add `HashFS.size()` method that returns the size of all files in
bytes.
- Add `HashFS.count()`/`HashFS.__len__()` methods that return the
count of all files.
- Add `HashFS.__iter__()` method to support iteration. Proxies to
`HashFS.files()`.
- Add `HashFS.__contains__()` method to support `in` operator. Proxies
to `HashFS.exists()`.
- Don\'t create the root directory (if it doesn\'t exist) until at
least one file has been added.
- Fix `HashFS.repair()` not using `extensions` argument properly.