Legand:
- not done
+ done
/ in progress

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+ Parameterize priority_size as a class variable using a class factory or similar.
+ Make it work for min and max, not just min
+ node retrieval
	+ find itself
	+ unit test
+ Add arbitrary node deletion
+ find the smallest
+ delete the smallest
+ find the largest
+ delete the largest
- Make it optionally keep the n smallest (largest) values only (probably via a wrapper class)
+ Improve the unit testing: should check for binary tree and heap invariants
+ change recursive methods to iterative?  Recursion stack overflow is a possibility in the current code.  Of course, it likely would be in an iterative
	implementation too - just not as soon.  Also, function/method calls are slow in python - an iterative implementation might help performance
+ reverse_iterator
+ make it work like a dictionary!
+ make remove_min and remove_max return the (key, value) they're removing
+ change exceptions.LookupError to exceptions.KeyError, for consistency with dict type
+ allow _treap_node to optionally be in cython, since it does the heavy lifting and doesn't need iterators