Better api generation

Things to consider:

• breaking changes (removed imports), so up minor version, because we are still not in 1.0

src/main/jastadd/backend/PrettyPrinting.jadd

@@ -33,10 +33,10 @@ aspect PrettyPrinting {
 
 aspect Utils {
   public String ASTNode.ind(int n) {
-    String s = "";
+    StringBuilder s = new StringBuilder();
     for (int i = 0; i < n; i++) {
-      s += "  ";
+      s.append("  ");
     }
-    return s;
+    return s.toString();
   }
 }