Support heap images for cakeml.

[ordinarymath]

It would be nice for cakeml to support heap images providing similar functionality to polyml heaps where you can save a heap state to disk and load the heap state.

[ordinarymath]

Allowing candle to use this would be explicit goal.

[xrchz]

What if anything would you want to be verified about this?

[ordinarymath]

It would a proof of equivalence between

1. run candle and "build" the base library. Export the heap state. Create a new instance of cakeml pass it the heap state as a param and continue.
2. run candle and "build the base library"