Bitwise `not` about bigints

[konsumlamm]

See also #51.

Your implementation wouldn't make sense, due to the way the BigInts are stored. For example, not 0'bi should be -1'bi and not 0xFFFFFFFF'bi, i.e. it should act as if the number was stored in 2's complement representation (which is what everyone uses for integers).

[dlesnoff]

This is indeed a duplicate of #51.

func `not`*(a: BigInt): BigInt = -a - 1

would be the simplest.
I think we could also use a cast to int32 on highest limb :

let leadingLimb = a.limbs[a.limbs.high]
let castLeadingLimb = cast[int32](not leadingLimb)
result.limbs[a.limbs.len-1] = abs(castLeadingLimb).uint32

and set the sign as :

result.isNegative = a.isNegative xor (castLeadingLimb > 0)