Improve Test harness for the Heap implementation

[scallyw4g]

Nearly everything in Bonsai is allocated on an Arena, for which there is a pretty fancy (and bulletproof) implementation. That said, I've started to want a heap in a few places. #7

The task at hand is to improve the test harness for the Heap implementation such that we're relatively confidant it's not buggy (or at least doesn't have any glaringly obvious bugs), so we can start using it in a select few cases.