In retrospect, I should've exposed ints to JS instead of entities.
Making this fix is already a TODO, but it will be a tedious change. For
now, this PR fixes two issues that were causing tiny but frequent memory
leaks, according to the emscripten memory leak widget.
(1) Do not include Entity in the list of classes that should only be
passed around by pointer.
(2) Clients should always call delete() on returned Entities.
f4004cf047