bake: Fix bugs introduced by bake optimizations, of course. - Make the execution stats JSON-serializable. - Re-add ability to differentiate between sources used during segment rendering and during layout rendering. Fixes problems with cache invalidation of pages that use other sources. - Make taxonomy-related stuff JSON-serializable.

*.pyc
*.egg-info
.cache
__pycache__
venv
tags
bower_components
node_modules
build/lib
util/messages/_cache
util/messages/_counter
dist
docs/_cache
docs/_counter
foodtruck/static
piecrust.egg-info
piecrust/__version__.py
.ropeproject