Save progress on multi-iteration tables

[blakeelias]

Prevent quadratic behavior where these tables get re-computed.
Compute the whole thing, while saving any intermediates along the way if they'll be needed.

Or save every power of 2 or something, so they can be combined easily.
(Can I create a simple method for combining two iterated tables to produce their lumped iteration?)

The whole story changes when doing time-varying environments anyway, since it's not just a matter of iterating the same transitions...