performance: Refactor how data is managed to reduce copying. * Make use of `collections.abc.Mapping` to better identify things that are supposed to look like dictionaries. * Instead of handling "overlay" of data in a dict tree in each different data object, make all objects `Mapping`s and handle merging at a higher level with the new `MergedMapping` object. * Since this new object is read-only, remove the need for deep-copying of app and page configurations. * Split data classes into separate modules.

--- a/piecrust/baking/worker.py	Sat Jun 27 22:28:32 2015 -0700
+++ b/piecrust/baking/worker.py	Sun Jun 28 08:22:39 2015 -0700
@@ -198,7 +198,7 @@
         try:
             page = fac.buildPage()
             page._load()
-            result.config = page.config.get()
+            result.config = page.config.getAll()
         except Exception as ex:
             logger.debug("Got loading error. Sending it to master.")
             result.errors = _get_errors(ex)