Implements random seed generation for fractal types using std::rand::Rand.

This commit motivates working on #1.

Signed-off-by: Atheriel <atheriel@gmail.com>
Implements seeds for 2D simplex noise.

This slows down the function greatly (184 -> 230 ns on my machine) so
perhaps there should be a way to avoid it.

The seeds for the doc-images example have also been modified slightly
to give nicer images for the documentation (in light of the change).

Closes #1.

Signed-off-by: Atheriel <atheriel@gmail.com>