feat(xanthous): Track the volume and density of item types

Allow the itemType raw to have density and volume fields, both of which
represent *intervals* of both density and volume (because both can
hypothetically vary a bit). The idea here is that when we're making
an *instance* of one of these items, we pick a random value in the
range.

Lots of stuff in this commit is datatype and typeclass instances to
support things like intervals being fields on datatypes that get
serialized to saved games - including a manual definition of Ord for
Item since Ord isn't well-defined for intervals

Change-Id: Ia088f2f75cdce9d00560297e5c269e3310b85bc3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3225
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/xanthous/src/Xanthous/App.hs

@@ -32,6 +32,7 @@ import           Xanthous.Data
                  , position
                  , Position
                  , (|*|)
+                 , Tiles(..)
                  )
 import           Xanthous.Data.App (ResourceName, Panel(..), AppEvent(..))
 import qualified Xanthous.Data.EntityMap as EntityMap
@@ -127,7 +128,7 @@ handleCommand (Move dir) = do
   collisionAt newPos >>= \case
     Nothing -> do
       characterPosition .= newPos
-      stepGameBy =<< uses (character . speed) (|*| 1)
+      stepGameBy =<< uses (character . speed) (|*| Tiles 1)
       describeEntitiesAt newPos
     Just Combat -> attackAt newPos
     Just Stop -> pure ()