Add menu support to the prompt system, and an "Eat" command that prompts
for an item to eat and eats the item the character specifies, restoring
an amount of hitpoints configurable via the item raw type.
Add a Brain class, which determines for an entity the set of moves it
makes every step of the game, and begin to implement that for gormlaks.
The idea here is that every step of the game, a gormlak will move
towards the furthest-away wall it can see.
Add a new "Item" entity, which pulls from the previously-existent
ItemType raw, and add a "PickUp" command which takes the (currently
*only*) item off the ground and puts it into the inventory.
