bignums and native back end

[Matthew Fluet]

> x * 1 = 1 * x = x
> x * ~1 = ~1 * x = ~x
> x * 0 = 0 * x = 0
> x + 0 = 0 + x = x
> x - 0 = x
> 0 - x = ~x
> x > x = x < x = false
> x >= x = x <= x = true
> equal(x, x) = true

x / 1 = x
x / ~1 = ~x

x - x = 0  (* probably rare *)