Bouncer contract implementation (#883)

* feat: implement bouncer contracts

* fix: rename Bouncer to SignatureBouncer

test/helpers/hashMessage.js

@@ -1,8 +0,0 @@
-import utils from 'ethereumjs-util';
-
-// Hash and add same prefix to the hash that ganache use.
-module.exports = function (message) {
-  const messageHex = Buffer.from(utils.sha3(message).toString('hex'), 'hex');
-  const prefix = utils.toBuffer('\u0019Ethereum Signed Message:\n' + messageHex.length.toString());
-  return utils.bufferToHex(utils.sha3(Buffer.concat([prefix, messageHex])));
-};