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.

syntax: glob
*.pyc
venv
tags
build/lib
build/messages/_cache
build/messages/_counter
dist
docs/_cache
docs/_counter
docs/bower_components
docs/node_modules
piecrust.egg-info
piecrust/__version__.py