since the object is not immediately removed, any other code may still
think an object is there, therefore leading to item duplication. This code
therefore sets the itemstring to '' after the object is picked up to avoid
such issues
On the lua side, notably minetest.env:<function>(<args>) should now
be replaced by minetest.<function>(<args>).
The old way is and will stay supported for a long time.
Also:
Update and clean up lua_api.txt (by celeron55)
Move EnvRef to lua and remove add_rat and add_firefly (by kahrl)
Add separate src/util/CMakeLists.txt, other minor fixes (by kahrl)
