hash: Improve double hashing

Instead of artificially introducing collisions in the step value by
replacing 0 with 1 (which causes the value 1 to have twice the
frequency of any other value), the step value can simply be computed
as an uniformly distributed value in the range [1, rehash], extremes
included.

This is safe because any step value smaller than the hash modulus is
co-prime with it, hence induces an orbit which includes every integer
in [0, size - 1].