Change to use simplex-noise instead of perlin

See also https://github.com/maxogden/voxel-simplex-terrain/blob/master/index.js
Fix crustLower initialization to 0
Save random
Add a second layer of noise to control how high the hills are ("roughness")

This is a larger scale noise, intended to give some more variety to the
terrain - instead of the same rolling hills everywhere, there are now
mountainous regions and flat plains (could use further refinement, though).
Move tree generation to populateChunk()
Fix chunks repeatedly populated
Change to use voxel-trees instead of voxel-forest

https://github.com/deathcap/voxel-trees