* Curing existing /nix
* Fixup macs
* Suggest an uninstall command if the binary is not present
* Fixup some nits
* Skip a not great suggestion
* Suggest a nice url
* wip: add more context to errors
* Add a bunch fo context
* Repair source handling
* Add remaining contexts
* Add some context, but some of it is not right...
* Tidy up contexts properly
* Get command errors working how I want
* Remove some debug statements
* Repair mac build
* Move typetag to Action
* newtypes!
* Fix doctest
* Add newline to end of `plan` json output
This causes some shells (like sh) to continue their prompt right after
the closing brace, which is kinda meh.
* Add newline to end of `receipt.json`
`cat /nix/receipt.json` has the same issue as the previous commit, in
that it will bleed into the prompt for some shells that don't insert an
implicit newline.
* Add newline to end of fixture json documents
* Enable deleting users and groups on Mac
* Scaffold user change
* Add a warning if it doesn't work in situations we expect it to not work
* Scaffold out doing group member ship -- maybe we need an AddGroup action
* AddUserToGroup action
* Update plans
* Improve messaging
* Nit in error message
* Repair some review nits
