core: Start of the big refactor for PieCrust 3.0. * Everything is a `ContentSource`, including assets directories. * Most content sources are subclasses of the base file-system source. * A source is processed by a "pipeline", and there are 2 built-in pipelines, one for assets and one for pages. The asset pipeline is vaguely functional, but the page pipeline is completely broken right now. * Rewrite the baking process as just running appropriate pipelines on each content item. This should allow for better parallelization.