internal: Get rid of the whole "sub cache" business. * Compute cache keys up front, so the cache directory is only chosen once. * Buffer up config variants to apply before loading the config. Makes it possible to cache variant-resulting configs, too. * Make a factory class to reuse the logic that creates the `PieCrust` object correctly for multi-process workers and such. * Add a test.

sudo: false
language: python
python:
  - "3.4"
install:
  - "pip install -r requirements.txt"
  - "pip install -r dev-requirements.txt"
script: py.test