This adds another test storage.FS implemenation. This can be used to
write/read containers and files from a local filesystem via calls to the
os pacakge.
This updates the fstest implementation of a storage.FS to support
opening sub containers and files in read-only mode.
Now when calling `SubContainer`:
- If using `storage.WithCreateFile`, an error is returned if the
container already exists.
- If not using `storage.WithCreateFile` and error is returned if the
container does *not* exist.
When calling `OpenFile`:
- on a container that was created with `storage.ReadOnly`, an error is
returned if attempting to open the file wth `storage.ReadWrite`.
- with `storage.WithCreateFile`, an existing file is replaced with a new
empty file, (ie "truncated").
- without `storage.WithCreateFile`, an error is returned if the file
does not exist.
This provides a Sha256Sums type to store the information from a
SHA256SUM file in memory for easy lookup of the sums of files. It can be
used when checking the checksums of individual files without needing to
go back to disk.
Timothy Messier
irenarindos
Jim