[Sumtree]: Implement prefix sum algorithm

[AlpinYukseloglu]

Closes: #76

What is the purpose of the change

This PR implements the prefix sum algorithm as defined by our spec. Specifically, it implements the function get_prefix_sum which gives the sum of all leaves up until the passed in ETAS value.

Testing and Verifying

[WIP] The intended testing strategy here is:
Testing strategy:

1. Create a sumtree
2. Run a bunch of insertions as in test_node_deletion_valid setup
3. Whenever an order is inserted with an ETAS <= our target for the test case, add its amount to the expected prefix sum
4. Run prefix sum at target ETAS on tree
5. Ensure equal to expected amount

Hopefully we can do a follow up PR to fuzz generate test cases here

[AlpinYukseloglu]

Note: all the changes in node.rs and test_node.rs are driveby cleanup from tree debugging w @crnbarr93

[crnbarr93]

Looks good! Comments were extremely useful in following the implementation. Two small nit changes otherwise no problems.