README: add build status
move chunk generation profiling into chunk_gen
improve terrain benchmark
factor out Perlin construction to TerrainGenerator
terrain: sample low resolution density data to create caves

Computing the density for every block was expensive. This change does it at 1/4
the resolution on each axis (64 times fewer total) and uses trilinear
interpolation to get a density value for individual blocks.
Merge pull request #1 from rlane/upsample-terrain

terrain generation optimization