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.

include README.rst
include CHANGELOG.rst
include LICENSE.rst
include requirements.txt
include dev-requirements.txt
recursive-include piecrust *.py mime.types
recursive-include piecrust/resources *
recursive-include tests *.py
#global-exclude .DS_Store
#global-exclude Thumbs.db
#global-exclude Desktop.ini
#global-exclude *.swp
#global-exclude *~
#global-exclude *.bak