feat(xan): Use Witherable in the prelude

Install the witherable library, expose it in the prelude, and update all
call sites that are broken by that change.

This is a really nice library, and basically the ideal abstraction layer
for what it does.

Change-Id: I640e099318c1ecce0ad483bc336c379698bdab88
Reviewed-on: https://cl.tvl.fyi/c/depot/+/725
Reviewed-by: glittershark <grfn@gws.fyi>

users/glittershark/xanthous/test/Xanthous/Messages/TemplateSpec.hs

@@ -62,7 +62,7 @@ test = testGroup "Xanthous.Messages.Template"
     ]
   ]
   where
-    genLiteral = filter (`notElem` ['\\', '{']) <$> arbitrary
+    genLiteral = pack . filter (`notElem` ['\\', '{']) <$> arbitrary
     parseCase name input expected =
       testCase name $ testParse template input @?= Right expected
     testParse p = over _Left errorBundlePretty . runParser p "<test>"