Change variable storage to a function that returns that variable (#332)

* Change variable storage to a function that returns that variable

* Improve naming

* 0.3.2

Author: openorclose

package.json

@@ -1,6 +1,6 @@
 {
   "name": "js-slang",
-  "version": "0.3.1",
+  "version": "0.3.2",
   "description": "Javascript-based interpreter for slang, written in Typescript",
   "author": {
     "name": "Source Academy",