use uint16 for treeArity in PollFactory.deploy

[exfinen]

In maci/contracts/contracts/Poll.sol,

since the type of treeArity is uint8,

        uint8 treeArity = 5;   // line 63

in

        require(   // line 73
            _maxValues.maxMessages <= treeArity ** _treeDepths.messageTreeDepth &&
            ...

treeArity ** _treeDepths.messageTreeDepth seems to overflow when _treeDepths.messageTreeDepth is greater than 3, i.e. 5^3 = 125 but 5^4 = 625 which is greater than 256, and that makes the require statement fail.

The type of treeArity should be a type that can store larger values without overflow.

[weijiekoh]

Thanks @exfinen! I pushed a new commit that uses uint256 for treeArity and casts messageTreeDepth and voteOptionTreeDepth to uin256 instead.