Murisi/masp based merkle tree

[murisi]

The Merkle tree implementation based on the Zcash/MASP implementation (from a month ago). Some notes:

• When --gas-report is used to measure gas usage, this implementation uses 30% less gas than the OpenZeppelin based implementation according to the benchmarks that were used to select the latter.
• This PR passes the differential Merkle tree fuzz tests (i.e. it computes identical roots to the OpenZeppelin implementation on a 1000 different Merkle trees).
• This PR only expands the Merkle tree when it is absolutely necessary. For this reason, a few of the tests had to be adjusted to expect trees with depths that are one less than the previous value.
• test_execute_mint_transfer_burn is the only new failing test. Its failure is a NonExistingRoot failure and is probably due it using Merkle roots from eager Merkle tree expansions.