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.

--exclude=build
--exclude=dist
--exclude=docs/_cache
--exclude=docs/_counter
--exclude=bower_components
--exclude=node_modules
--exclude=docs/raw
--exclude=*.egg-info
--exclude=__pycache__
--exclude=__tmpfs__