Roadmap to v2

Many repos have a roadmap to v1.
This plan goes a little further.

The idea is:

ChainRules/ChainRulesCore v1:

• It pragmatically works.
• It is completely usable, and is in use by multiple AutoDiff systems by the time we tag v1.
• But it might play fast and lose with the math, like it will probably keep something like the current defintion of AbstractDifferential, which conflates the idea of scaling and adding.
• It has some edge cases that we just bail out on and don't handle representing. This may include complex deriviatives, and it probably will include mutation/nonpure functions.
• It will support Julia 1.0

ChainRules/ChainRulesCore v2:

• The math will be tighter
• After a while of use in public we will have identified the actual edge cases that matter of things we can't represent. and it will be enhanced to support those
• It may not support julia 1.0, e.g. might depend on features added in 1.3

This roadmap needs some refining,
but this was the big picture @willtebbutt and I were talking about

No files, tests, or entry points are named. Start by reviewing the existing ChainRules/ChainRulesCore v1 scope and Julia-version support, then refine the broad v2 goals into concrete changes and acceptance criteria before implementation.