Add `neg` to negate numbers

The unary minus operator will never be implemented because it would be ambiguous due to #92: `-3`, or `- 3`, is meant to return a function that subtracts 3 rather than the number -3.

Implement a global `neg` function to replace the unary minus operator:

``` lambda
-3     # closure
neg 3  # -3
```

It will not be a keyword.
