Function asOrdinal

Replaces all tree nodes with their unique ordinal. The order is depth-first post-order, so that the root node value is the maximum. For example a tree shaped so:

 ┬?
 ├─?
 └┬?
  ├─?
  └─?
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Will become this tree:

 ┬5
 ├─1
 └┬4
  ├─2
  └─3
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