feat(xanthous): Fistfighting builds knuckle calluses

2000 ticks after the character damages their fists by hitting something,
the character now develops calluses on their fists (scaled by *how*
damaged they've become) that reduce the chance of them receiving
additional damage from hitting things - up to a max of 5, which
prevents *all* damage from fistfighting.

This is all tracked in a new "Knuckles" struct in a new "Body" struct on
the character datatype, which manages stepping itself forward as part of
the Brain impl on the character.

Change-Id: Ica269f16fb340fb25900d2c77fbad32f10c00be2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3222
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/xanthous/test/Xanthous/UtilSpec.hs

@@ -2,6 +2,7 @@ module Xanthous.UtilSpec (main, test) where
 
 import Test.Prelude
 import Xanthous.Util
+import Control.Monad.State.Lazy (execState)
 
 main :: IO ()
 main = defaultMain test
@@ -25,4 +26,12 @@ test = testGroup "Xanthous.Util"
     [ testProperty "takeWhileInclusive (const True) ≡ id"
       $ \(xs :: [Int]) -> takeWhileInclusive (const True) xs === xs
     ]
+  , testGroup "endoTimes"
+    [ testCase "endoTimes 4 succ 5"
+      $ endoTimes (4 :: Int) succ (5 :: Int) @?= 9
+    ]
+  , testGroup "modifyKL"
+    [ testCase "_1 += 1"
+      $ execState (modifyKL _1 $ pure . succ) (1 :: Int, 2 :: Int) @?= (2, 2)
+    ]
   ]