genesisMint gas costs can be reduced by caching genesisCounter

uint256 cachedGenesisCounter = genesisCounter;
for (uint i; i < _to.length; ++i) {
    _mint(_to[i], _ids[i]);
    isGenesis[_ids[i]] = true;
    cachedGenesisCounter++;
}
genesisCounter = cachedGenesisCounter;