Remove --enable-unstable #3068
Description
Proposed change: Deprecate and remove --enable-unstable. For legacy arguments like --cbmc-args, --visualize, --function and other unstable arguments, the proposal was to create a unstable feature -Z unstable-options.
For things that are only added for debug / development purpose, I propose we add something like Z dev-features.
Motivation: This is the missing part of the Unstable APIs RFC.