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/page.py	Sat Jun 27 22:28:32 2015 -0700
+++ b/piecrust/page.py	Sun Jun 28 08:22:39 2015 -0700
@@ -284,7 +284,7 @@
 
     # Save to the cache.
     cache_data = {
-            'config': config.get(),
+            'config': config.getAll(),
             'content': json_save_segments(content)}
     cache.write(cache_path, json.dumps(cache_data))