Add `neg` to negate numbers

[71104]

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:

-3     # closure
neg 3  # -3

It will not be a keyword.