Add support for multiple levels

Add a data structure, based on the zipper comonad, which provides
support for multiple levels, each of which is its own entity map. The
current level is provided by coreturn, which the `entities` lens has
been updated to use. Nothing currently supports going up or down levels
yet - that's coming next.

package.yaml

@@ -26,6 +26,7 @@ dependencies:
 - checkers
 - classy-prelude
 - comonad
+- comonad-extras
 - constraints
 - containers
 - data-default
@@ -48,6 +49,7 @@ dependencies:
 - MonadRandom
 - mtl
 - optparse-applicative
+- pointed
 - random
 - random-fu
 - random-extras
@@ -59,6 +61,7 @@ dependencies:
 - stache
 - semigroupoids
 - tomland
+- text
 - text-zipper
 - vector
 - vty