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.

--exclude=build
--exclude=dist
--exclude=docs/_cache
--exclude=docs/_counter
--exclude=bower_components
--exclude=node_modules
--exclude=docs/raw
--exclude=*.egg-info
--exclude=__pycache__
--exclude=__tmpfs__