WIP: handle zero_tangent from cyclic data structures v2 Via premapping

[oxinabox]

This is an alternative to https://github.com/JuliaDiff/ChainRulesCore.jl/pull/654 I think it is more promising and should minimize the type instability outside of cyclic locations

Basically here is the idea:

1. Identify all objects that have multiple references to them
2. Go through and construct tangents for everything;
3. save the tangents for things with >1 reference into an IdDict keyed by primal
4. when constructing tangent fields for things that have >1 reference to them, initially treat them as per undef elements, so set the tangent value as also undef or to a ZeroTangent
5. go back through the tangent object and set the values for things with multiple references

This will also handle aliases outside of loops correctly.