We now have a notion of libraries that we load after app code, to
prevent them from getting overridden.
Should I just load all libraries after the app? There might be value in
allowing apps to override library functions. Disallowing that too much
may be going against Lua's dynamic nature.
We aren't actually secure as the previous commit said. The hole here is
that you can't override start_writing by typing in 'start_writing' into
the big picture. However you _can_ override it by typing in _anything
else_.