function stringToAddress(string memory _a) public pure returns (address) {
        bytes memory tmp = bytes(_a);
        uint160 iaddr = 0;
        uint160 b1;
        uint160 b2;
        for (uint i = 2; i < 2+2*20; i += 2) {
            iaddr *= 256;
            b1 = uint160(tmp[i]);
            b2 = uint160(tmp[i+1]);
            if ((b1 >= 97)&&(b1 <= 102)) b1 -= 87;
            else if ((b1 >= 48)&&(b1 <= 57)) b1 -= 48;
            if ((b2 >= 97)&&(b2 <= 102)) b2 -= 87;
            else if ((b2 >= 48)&&(b2 <= 57)) b2 -= 48;
            iaddr += (b1*16+b2);
        }
        return address(iaddr);
    }

VehicleRegistry1.sol:266:18: TypeError: Explicit type conversion not allowed
from "bytes1" to "uint160".
            b1 = uint160(tmp[i]);
                 ^-------------^
VehicleRegistry1.sol:267:18: TypeError: Explicit type conversion not allowed from "bytes1" to "uint160".
            b2 = uint160(tmp[i+1]);
                 ^---------------^