(Un)Locking keys for lockable objects!

This mod introduces a single new item in the inventory - a skeleton key. The key can be generated from a steel ingot, and then used on a lockable chest, a lockable door (or a lockable trapdoor, if you have my other mod). Any existing locking chest, or door will work.
Once the skeletonkey is used on a lockable item, it is converted to a node-specific key. The node-specific key is yellow, and will from there on only ever fit that one chest or door.
The key can then be given to a friend or other player, allowing them access to the chest contents or allowing them to open and close doors that they otherwise could not. They will have to wield the key and right-click the door or chest to interact with the item.
The key can be recycled in an oven, smelting it back to an ingot.
If the owner of the lockable node destroys that object, and puts a new lockable object back in the same position, the old key will not open the new lockable object. Keys are really specific to the chest they were used, and are useless afterwards.
An issue is currently that there's no way to distinguish keys from each other. oops. Better keep your keys in order. I am working on a method to rename items that will resolve this in the future.
Code: this is a fork from minetest_game:
https://github.com/sofar/minetest_game/tree/keys
License: equal to minetest_game (derivative)