Implement the start of creature AI

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.

src/Xanthous/Data/EntityMap.hs

@@ -14,6 +14,7 @@ module Xanthous.Data.EntityMap
   , insertAt
   , insertAtReturningID
   , fromEIDsAndPositioned
+  , toEIDsAndPositioned
   , atPosition
   , atPositionWithIDs
   , positions
@@ -101,6 +102,14 @@ instance Semigroup (EntityMap a) where
 instance Monoid (EntityMap a) where
   mempty = emptyEntityMap
 
+instance FunctorWithIndex EntityID EntityMap
+
+instance FoldableWithIndex EntityID EntityMap
+
+instance TraversableWithIndex EntityID EntityMap where
+  itraversed = byID . itraversed . rmap sequenceA . distrib
+  itraverse = itraverseOf itraversed
+
 emptyEntityMap :: EntityMap a
 emptyEntityMap = EntityMap mempty mempty 0
 
@@ -183,6 +192,9 @@ fromEIDsAndPositioned eps = newLastID $ alaf Endo foldMap insert' eps mempty
       .~ fromMaybe 1
          (maximumOf (ifolded . asIndex) (em ^. byID))
 
+toEIDsAndPositioned :: EntityMap a -> [(EntityID, Positioned a)]
+toEIDsAndPositioned = itoListOf $ byID . ifolded
+
 positions :: EntityMap a -> [Position]
 positions = toListOf $ byPosition . to keys . folded