I'm not aware of efforts to begin implementing this yet. It will be a
very fundamental change and will likely be made far more complicated
by the on-going flow versioning work and performance improvements
happening. You might wish to delve deep into that flow versioning
feature proposal, JIRA, and current PR to get a full understanding of
what is happening there. For a feature this foundational it is
probably a good idea to more fully detail the feature proposal to
cover some design or implementation ideas.
I am definitely a big fan of this concept and know it will be a big
help to folks for providing reusable 'functions' that can be invoked
throughout the flow.