Extract function and node specific logic from TD3

[sim642]

Part of issue #545.

This is a quick idea I had to move all the incrementally changed functions stuff out of TD3. The point is to expose separate constraint system incremental data to TD3, which just lists the unknowns to obsolete, delete and reluctantly resolve. These lists are populated by FromSpec itself, which is the last thing that still should know about the specifics of the constraints.
With this change it's actually unnecessary for the solver to even have access to iter_vars from #549 because FromSpec uses it directly to do the conversion.

This is currently on top of #391 although something similar could be done on master as well. Although the annoyance is that it would have to be essentially redone completely there since the changes code in TD3 is changed significantly by #391, particularly by #549.

TODO

• Remove iter_vars access from solver.
• Fix server non-relifting in TD3.
• Benchmark performance change due to intermediate lists?

[jerhard]

Looks good!