IF: Create new efficient incremental Merkle tree

Resolves #2333

This PR provides a new Merkle tree implementation which does not unnecessarily store extra digests and does not do extra hashes.

Previous versions are renamed: incremental_merkle_tree_legacy, calculate_merkle_legacy. New versions are named: incremental_merkle_tree, calculate_merkle.

Note: Even though the files have been renamed incremental_merkle_legacy.hpp and merkle_legacy.hpp, the implementation has not been modified (besides remoning the canonical template parameter).

New `incremental_merkle_tree`

does not store extra hashes (root matches calculate_merkle)
provides the root on demand, does not compute it on append().
is about 20x faster than incremental_merkle_tree_legacy.
storage is minimal. Just one uint64_t and a vector<digest_type> whose size() is std::popcount(num_digest_appended).

New `calculate_merkle`

does not store extra hashes. (root matches incremental_merkle_tree).
is about 25% faster than calculate_merkle_legacy without multithreading
supports internal multithreading for computing roots of collections whose size() > 4096, in which case it is 2x to almost 4x faster (2x for sequences of at least 10k digests, 4x for very large sequences).
doesn't mutate the passed deque<digest_type>.

New performance tests

This PR adds two performance tests comparing the legacy and new merkle tree implementations

./unittests/unit_test "--run_test=merkle_tree_tests/perf_test_one_large"
./unittests/unit_test "--run_test=merkle_tree_tests/perf_test_many_small"

New consistency test

This PR adds a test checking consistency between incremental_merkle_tree and calculate_merkle when adding from 1 to 1000 digests:

./unittests/unit_test "--run_test=merkle_tree_tests/consistency_over_large_range"

AntelopeIO / leap