use a Dictionary object instead of plain object for hashes

to mitigate the `__proto__` issue

related to #30

lib/utils.js

@@ -244,3 +244,23 @@ function makePredicate(words) {
     }
     return new Function("str", f);
 };
+
+function Dictionary() {
+    this._values = Object.create(null);
+};
+Dictionary.prototype = {
+    set: function(key, val) { return this._values["$" + key] = val, this },
+    get: function(key) { return this._values["$" + key] },
+    del: function(key) { return delete this._values["$" + key], this },
+    has: function(key) { return ("$" + key) in this._values },
+    each: function(f) {
+        for (var i in this._values)
+            f(this._values[i], i.substr(1));
+    },
+    map: function(f) {
+        var ret = [];
+        for (var i in this._values)
+            ret.push(f(this._values[i], i.substr(1)));
+        return ret;
+    }
+};