Previous state/frame awareness

Being able to access previous frame info is a key part of other node-based workflows (looking at you houdini) - comparing current value with previous value has a number of practical applications :

1. making constraints more stable (solves the infamous copyrot flipping past 180 degrees)
2. creating simple dynamics through drivers (user can derive velocity from comparison between current and previous state and use that information in a rig, to make an automatic wiggling tail, or tentacles)
3. in the future, allows to create own simple solvers for simulations (in the context of object nodes, fluid sim or particle nodes, etc)

Hadrien

(added more obvious example cases)