* make 64-bit build more memory-efficient (issue213)
* log maximum open-list size and compare to number of generated states (msg5844)
* use more aggressive duplicate elimination if open-list size is a bottleneck 
  (msg5844)
* Consider deque instead of vector for IntHashSet because it can deal with
fragmented memory.
* If we use a deque for IntHashSet, consider growing it in place when resizing.
(This complicates rehashing.)
* Cache heuristic values by default only for a subset of the heuristics 
(issue579)
* Reduce memory usage of causal graph heuristic cache (issue838)

consistency edit: use "*" for all bullet points in summary rather than "*" and "-".

Add issue838 to summary.

I think the last point of the summary confounded two separate points, so I've
split it into two. (Working with non-contiguous memory is an advantage of deques
that we can get without changing anything about the rehashing. So there is an
easy change and a hard change. Either change can be individually beneficial in
terms of memory usage. I find it hard to predict what they would mean for
runtime, but the second change certainly sounds like it might incur a runtime
penalty.)

During the discussion about issue213 we decided to open a meta issue for making 
the planner more memory-efficient.