Add a drop command

Add a drop command, bound to 'd', which prompts the character for an
item in their inventory, removes it from the inventory, and places it on
the ground. Along the way I had to fix a bug in the
`EntityMap.atPosition` lens, which was always appending to the existing
entities at the position on set, without removing the entities that were
already there - the rabbit hole of quickchecking the lens laws here also
lead to replacing the target of this lens with a newtype called
`VectorBag`, which ignores order (since the entitymap makes no
guarantees about order of entities at a given position).

xanthous.cabal

@@ -4,7 +4,7 @@ cabal-version: 1.12
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 0476b4307dfceb20b9358ca2e6f78c753e3e0a4ae60c6faed54528f6a9c0dc5c
+-- hash: ae5b84ec168dd61b715e874bcb49579697873b164c43027a776dda725dfdffbf
 
 name:           xanthous
 version:        0.1.0.0
@@ -37,6 +37,7 @@ library
       Xanthous.Data.EntityChar
       Xanthous.Data.EntityMap
       Xanthous.Data.EntityMap.Graphics
+      Xanthous.Data.VectorBag
       Xanthous.Entities.Character
       Xanthous.Entities.Creature
       Xanthous.Entities.Draw.Util
@@ -96,7 +97,6 @@ library
     , megaparsec
     , mtl
     , optparse-applicative
-    , parallel
     , quickcheck-instances
     , quickcheck-text
     , random
@@ -125,6 +125,7 @@ executable xanthous
       Xanthous.Data.EntityChar
       Xanthous.Data.EntityMap
       Xanthous.Data.EntityMap.Graphics
+      Xanthous.Data.VectorBag
       Xanthous.Entities.Character
       Xanthous.Entities.Creature
       Xanthous.Entities.Draw.Util
@@ -183,7 +184,6 @@ executable xanthous
     , megaparsec
     , mtl
     , optparse-applicative
-    , parallel
     , quickcheck-instances
     , quickcheck-text
     , random
@@ -248,7 +248,6 @@ test-suite test
     , megaparsec
     , mtl
     , optparse-applicative
-    , parallel
     , quickcheck-instances
     , quickcheck-text
     , random