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)

Nodes in this group will be dropped as items if the node under them or the node in the wallmounted direction is not walkable.

Add after_destruct and cache the existence of on_construct, on_destruct and after_destruct for quick skipping when a node does not have them