Revert "refactor: remove "equal" file"

This reverts commit f9fcc504a4.

lib/compile/equal.js

@@ -0,0 +1,3 @@
+'use strict';
+
+module.exports = require('fast-deep-equal');

spec/custom.spec.js

@@ -2,7 +2,7 @@
 
 var getAjvInstances = require('./ajv_instances')
   , should = require('./chai').should()
-  , equal = require('fast-deep-equal')
+  , equal = require('../lib/compile/equal')
   , customRules = require('./custom_rules');