Use menus for combat and picking up items

Refactor a bunch of stuff around to allow for polymorphically surfacing
an EntityChar for all entities, and use this to write a generic
`entityMenu` function, which generates a menu from the chars of a list
of entities - and use that to fully implement (removing `undefined`)
menus for both attacking and picking things up when there are multiple
entities on the relevant tile.

package.yaml

@@ -25,6 +25,7 @@ dependencies:
 - brick
 - checkers
 - classy-prelude
+- comonad
 - constraints
 - containers
 - data-default