Stubbing out a few more items.
Fix an egregious bug in my Perlin impl, and compensate for the fallout.

Haha, oops, I multiplied by sqrt(x² + y² + ...) instead of dividing, so
my gradients weren't actually unit vectors at all, fuck me.

But fixing that has produced the very overt central clustering effect
Perlin has, so I'm using a second round of the s-curve function to try
to make the extremes a little more common.