Implemented visit_Return for freeing variables

[anutosh491]

Fixes #2411

We need to take care of two things

1. If we have a function returning something, we would like to free all local basic variables before returning
2. As we are retuning the _lpython_return_variable I don't think so we need to free that.

Now we would have something like

                                    (SubroutineCall
                                        2 basic_free_stack
                                        2 basic_free_stack
                                        [((Var 3 stack0))]
                                        ()
                                    )
                                    (SubroutineCall
                                        2 basic_free_stack
                                        2 basic_free_stack
                                        [((Var 3 stack1))]
                                        ()
                                    )
                                    (Return)]
                                    (Var 3 _lpython_return_variable)

Where stack0 and stack1 might be some local variables used inside the function, then followed by the Return statement.

[Thirumalai-Shaktivel]

I think it's fine, thank you!

[certik]

Can you add a test?

Why not calling the function to subroutine pass?

[certik]

I think it's fine.

[certik]

This is fixing memory leaks.