share internals of R.arity and R.nAry

I don't see a reason to create some of the arity fixers up front, since each one is created at most once anyway.

The idea is to avoid false positives. Consider the current behaviour:

```javascript
> R.nAry('toString', R.I)
'[object Undefined]'
```

On this branch, the behaviour is more sensible:

```javascript
> R.nAry('toString', R.I)
RangeError: Invalid array length
```