double speed of keyFile

Optimising for the common case of nothing needing to be escaped, from 5.434 μs
to 1.727 μs.

In the uncommon case, it only runs around 70 ns slower.

Annex/Locations.hs

@@ -514,7 +514,11 @@ keyFile :: Key -> FilePath
 keyFile = fromRawFilePath . keyFile'
 
 keyFile' :: Key -> RawFilePath
-keyFile' = S8.concatMap esc . serializeKey'
+keyFile' k = 
+	let b = serializeKey' k
+	in if any (`S8.elem` b) ['&', '%', ':', '/']
+		then S8.concatMap esc b
+		else b
   where
 	esc '&' = "&a"
 	esc '%' = "&s"