Solidity - convert uint256 array to bytes

Converter.sol

contract Converter {
  function u256a_tobytes(uint256[] memory arr) public pure returns (bytes memory) {
        bytes memory b = new bytes(arr.length*32);
        for (uint256 i = 0; i < arr.length; i++) {
            uint256 x = arr[i];
            uint256 j = 32+(i*32);
            assembly {
                mstore(add(b, j), x)
            }
        }
        return b;
    }
}