MerkleTreeSha256.sol Gas optimization

[klmcdon]

Currently, the global array "leaves" is defined dynamically as "bytes32[] leaves", and then explicitly set to size nbLeaves in the constructor. The entries of leaves are also manually initialized to their null value using a for loop in the constructor. This explicit initialization increases the gas cost of deployment by a non-trivial amount (especially for deep trees).

An alternative approach is to define a local array of fixed size nbLeaves, that is automatically initialized to its null value, in the constructor and then set the global array equal to this fixed-length array. E.g.

contract MerkleTreeSha256 {
...
bytes32[] leaves

constructor(uint treeDepth) public {
    ...   
    bytes32[] memory leavesBuilder = new bytes32[](nbLeaves);
    leaves = leavesBuilder;
}
...

}
Remix tests indicate that this reduces the cost of deployment.

[rrtoledo]

Done in fix-todo.