support different variable names

Two functions with different variable names but same semantics should result in the same checksum. Example:

```
def some_strange_looking_function(param1, callback_fn):
    tail = param1[-1]
    return callback_fn(tail)
```

```
def same_function_but_with_different_variable_names(xxx, callback_yyy):
    trail = xxx[-1]
    return callback_yyy(trail)
```
