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.

# This is a utility module that can be used with any WSGI-compatible server
# like Werkzeug or Gunicorn. It returns a WSGI app for serving a PieCrust
# website located in the current working directory.
import os
from piecrust.wsgiutil import get_app


root_dir = os.getcwd()
app = get_app(root_dir)
# Add this for `mod_wsgi`.
application = app