* core: partially implement sumtree; appending and pruning work
* core: add simple merkle-root-sum-calculating function.
* core: implement replacement in sumtree
* core: implement de/serialization for sumtree
* core: make sumtree index use hashes rather than data
* core: prevent double-adds to sumtrees
* core: replace sumtree's separate sums with a Summable trait
* core: make SumTree::contains() return the index if an element exists
Merope Riddle
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