Enable use of the mutable head extension

[jcoyne]

https://ocfl.github.io/extensions/0005-mutable-head.html

[mjgiarlo]

Hmmm, do you think this is in conflict with #14?

[jcoyne]

I have no idea. I just saw this for the first time and thought we might want to look into it.

[mjgiarlo]

No worries, we're all learning at a rapid clip here. This is not in the spec, but it is in the OCFL 1.1 implementation notes: https://ocfl.io/1.1/implementation-notes/#segregating-objects-in-flight

Based on my quick read, using a workspace gets you out of the business of mucking about in the OCFL object root unless e.g. you're writing a new version? Which seems like it's a different way to operate than a mutable HEAD.