Heap uses the old `rank` design

[MathisGD]

https://github.com/morpho-dao/morpho-data-structures/blob/c5846fa86664410f06bc739d83121671854d83fe/contracts/Heap.sol#L12

It should be changed to the index design, to match HeapOrdering.

[MathisGD]

The tests could also be improved
#68 (comment)

[QGarchery]

Specifically: we could improve tests by using invariants instead of randomized tests. See the test testFullHeapSort