Word{8,32}.neg

[Stephen Weeks]

> We might now begin considering expanding the set of
> primitives to include operations that can be expressed efficiently at the
> instruction level, even if they don't have a corresponding counterpart in
> the basis-library.

Makes sense.  I went ahead and added Word{8,32}_neg.

> The motivating example in this case being a
> Word{8,32}_neg primitive.  I'd also argue for Word{8,32} rotate
> primitives, but I'll hold off on that until I actually compare the running
> times of a rotate to the pair of shifts and an or.

OK.  Lemme know if you want 'em.