@0x/contracts-test-utils: Add hexConcat() in hex_utils.ts.

contracts/test-utils/src/hex_utils.ts

@@ -0,0 +1,8 @@
+import * as ethUtil from 'ethereumjs-util';
+
+/**
+ * Concatenate all arguments as a hex string.
+ */
+export function hexConcat(...args: Array<string | number | Buffer>): string {
+    return ethUtil.bufferToHex(Buffer.concat(args.map(h => ethUtil.toBuffer(h))));
+}