Implement messages

Implement messages almost the same as in the Rust version, only with
YAML instead of TOML this time, and a regular old mustache template
instead of something handrolled. Besides that, pretty much everything
here is the same.

test/Xanthous/OrphansSpec.hs

@@ -0,0 +1,31 @@
+{-# LANGUAGE BlockArguments #-}
+module Xanthous.OrphansSpec where
+
+import Test.Prelude
+import Xanthous.Orphans
+import Text.Mustache
+import Text.Megaparsec (errorBundlePretty)
+
+import Xanthous.Orphans ()
+
+main :: IO ()
+main = defaultMain test
+
+test :: TestTree
+test = testGroup "Xanthous.Orphans"
+  [ localOption (QuickCheckTests 50)
+  . localOption (QuickCheckMaxSize 10)
+  $ testGroup "Template"
+    [ testProperty "ppTemplate / compileMustacheText " \tpl ->
+        let src = ppTemplate tpl
+            res :: Either String Template
+            res = over _Left errorBundlePretty
+                $ compileMustacheText (templateActual tpl) src
+            expected = templateCache tpl ^?! at (templateActual tpl)
+        in
+          counterexample (unpack src)
+          $ Right expected === do
+            (Template actual cache) <- res
+            maybe (Left "Template not found") Right $ cache ^? at actual
+    ]
+  ]