Bouncer contract implementation (#883)

* feat: implement bouncer contracts

* fix: rename Bouncer to SignatureBouncer

test/helpers/sign.js

@@ -0,0 +1,22 @@
+import utils from 'ethereumjs-util';
+
+/**
+ * Hash and add same prefix to the hash that ganache use.
+ * @param {string} message the plaintext/ascii/original message
+ * @return {string} the hash of the message, prefixed, and then hashed again
+ */
+export const hashMessage = (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])));
+};
+
+// signs message using web3 (auto-applies prefix)
+export const signMessage = (signer, message = '', options = {}) => {
+  return web3.eth.sign(signer, web3.sha3(message, options));
+};
+
+// signs hex string using web3 (auto-applies prefix)
+export const signHex = (signer, message = '') => {
+  return signMessage(signer, message, { encoding: 'hex' });
+};