Make sure in-flight changes are done in workspaces

[mjgiarlo]

This work requires adding support for workspaces in the gem and then using workspaces to make changes to files. For inspiration, review other production OCFL libraries, e.g.:

• https://github.com/OCFL/ocfl-java/blob/main/docs/USAGE.md#ocfl-repository-configuration
• https://github.com/inveniosoftware/ocflcore#quick-start

[mjgiarlo]

I am now thinking that the way we do this is via the mutable head extension (#28), so we may want to pursue this sooner rather than later.