align compression func with doc description

[yueawang]

Make some changes according to doc description:

specifically, x is negative if the (b − 1)-bit encoding of x is lexicographically larger than the (b − 1)-bit encoding of −x.

and

This encoding is also used to define a b-bit encoding of each element (x, y) ∈ E as a b-bit string (x, y), namely the (b − 1)-bit encoding of y followed by a sign bit; the sign bit is 1 if and only if x is negative.

[Brechtpd]

Looks correct to me.

Seems like the ethsnarks code depends on x being odd/even for the compression instead of positive/negative but I don't know what the theory behind that would be and I don't know if that would work. I remember trying a couple of things out that didn't work out, not sure if this is one of them. I guess safer in any case to just follow the paper.

[yueawang]

Let me ask Harry for the reason. HarryR#165