WIP: numba arithmetic

first we have

```
ipdb> expr3
(amount + id) / amount
```
then

```
ipdb> expr3
_ / amount
```

then because above i've returned a `BinOp`, we have this:

```
ipdb> t
_ / amount
ipdb> kwargs
{'scope': {amount: array([ 100, -200,  300,  400, -500]), _: <_ast.BinOp object at 0x10faef090>}}
```