Update the way variables' values are computed

Example, this test 

```
def a: int = 59 + 1
def b: int = 9

output(a)
```

returns

```
Factor {
    token: Token {
        token: Variable(
            "a",
        ),
        line_number: 3,
        col_number: 7,
    },
}
() VisitResult { token: Variable("a") }
```

Since there is no implementation for Variable's visit method