REF: ops.missing
- Move arithmetic-centric functions from core.missing to core.ops.missing.
- module-level docstring giving the motivation for these functions
- tiny optimization:

```
if op in [operator.floordiv, operator.truediv, getattr(operator, "div", None)]:
    result = mask_zero_div_zero(left, right, result)
```

becomes

```
if op is operator.floordiv:
    result = mask_zero_div_zero(left, right, result)
```

because in py3 numpy behaves the way we want for truediv.
thanks @jbrockmendel 