Prototype to capture variables so that they can be used in an inner lambda

[metoule]

The idea is:

• use a dictionary to store captured variables (= parameters from all levels of lambda expressions)
• when parsing an expression, first assign the captured parameter to the dictionary

For example, it we parse x + 5, and call f(2) we convert it to:

dict["x"] = 2;
return dict["x"] + 5;

Fix #212