bake: Improve render context and bake record, fix incremental bake bugs. * Used sources and taxonomies are now stored on a per-render-pass basis. This fixes bugs where sources/taxonomies were used for one pass, but that pass is skipped on a later bake because its result is cached. * Bake records are now created for all pages even when they're not baked. Record collapsing is gone except for taxonomy index pages. * Bake records now also have sub-entries in order to store information about each sub-page, since some sub-pages could use sources/taxonomies differently than others, or be missing from the output. This lets PieCrust handle clean/dirty states on a sub-page level.