This is done by making fs.Config private and attaching it to the
context instead.
The Config should be obtained with fs.GetConfig and fs.AddConfig
should be used to get a new mutable config that can be changed.
This was started by Fionera, finished off by Laura with fixes and more
docs from Nick.
Co-authored-by: Fionera <fionera@fionera.de>
Co-authored-by: Nick Craig-Wood <nick@craig-wood.com>