Bring over MTX constant from abandoned PR

Author: area

contracts/common/MetaTransactionMsgSender.sol

@@ -5,10 +5,13 @@ import { DSMath } from "../../lib/dappsys/math.sol";
 
 abstract contract MetaTransactionMsgSender is DSMath {
   bytes32 constant METATRANSACTION_FLAG = keccak256("METATRANSACTION");
+  uint256 constant METATRANSACTION_DATA_MIN_LENGTH = 32 + 20;
+  // Where 32 is the length of METATRANSACTION_FLAG in bytes
+  // Where 20 is the length of an address in bytes
 
   function msgSender() internal view returns (address payable sender) {
     uint256 index = msg.data.length;
-    if (msg.sender == address(this) && index >= 52) {
+    if (msg.sender == address(this) && index >= METATRANSACTION_DATA_MIN_LENGTH) {
       bytes memory array = msg.data;
       bytes32 flag;
       assembly {