Provide an explicit Cache API

[Morendil]

Following discussions on #871 (on here and IRL with @fpagnoux) we've identified a few "sticks" that should be moved out of the way before implementing some performance optimizations that seem quite promising (split-and-combine, and possibly later taking advantage of static analysis to prune the computation tree ahead of time).

These include #885 and #886 to warm up. The next significant bit is to make the caching API explicit.

[bonjourmauko]

A tl;dr of previous efforts was to:

1. Move the cache from the variables to the simulation.
2. Have a way of rewinding or itemising calculations.
3. Separate cache and storage layers.