Evaluate booleans and strings

src/evaluator.js

@@ -14,7 +14,9 @@ module.exports = class Evaluator {
 
   evalNode(node, env) {
     switch (node.constructor) {
+      case AST.Boolean:
       case AST.Number:
+      case AST.String:
         return node
       case AST.Identifier:
         return env.get(node.name)