Change contract hash combination to use summation

[ureeves]

Make use of summation instead of hashing to combine two different contract hashes in the calculation of the root. It entails a trade-off between assuring the ordering/structure of the Hamt and performance.

[ureeves]

For context here's a comparison from the bench:

root calculation (hash): [628.87 ns 629.37 ns 629.89 ns]
root calculation (sums): [38.124 ns 38.145 ns 38.168 ns]

[ureeves]

Totally irrelevant since a long time