core: Continue PieCrust3 refactor, simplify pages. The asset pipeline is still the only function pipeline at this point. * No more `QualifiedPage`, and several other pieces of code deleted. * Data providers are simpler and more focused. For instance, the page iterator doesn't try to support other types of items. * Route parameters are proper known source metadata to remove the confusion between the two. * Make the baker and pipeline more correctly manage records and record histories. * Add support for record collapsing and deleting stale outputs in the asset pipeline.

--- a/piecrust/serving/middlewares.py	Wed May 17 00:11:48 2017 -0700
+++ b/piecrust/serving/middlewares.py	Sun May 21 00:06:59 2017 -0700
@@ -89,8 +89,8 @@
         if not found:
             return NotFound("No such page: %s" % page_path)
 
-        ctx = DataBuildingContext(req_page.qualified_page,
-                                  page_num=req_page.page_num)
+        ctx = DataBuildingContext(req_page.page,
+                                  sub_num=req_page.sub_num)
         data = build_page_data(ctx)
 
         var_path = request.args.getlist('var')