Revert 33ee15d56f.

Revert to the older faster toChar. I would have just removed the
old commit from history but it seems that would call for a rebase
which I hear is a bad idea in a public branch. Thus this commit.

yesod-core/Yesod/Internal/Request.hs

@@ -75,7 +75,10 @@ parseWaiRequest' env session' key' gen = Request gets'' cookies' env langs' nonc
 randomString :: RandomGen g => Int -> g -> String
 randomString len = take len . map toChar . randomRs (0, 61)
   where
-    toChar i = (['A'..'Z'] ++ ['a'..'z'] ++ ['0'..'9']) !! i
+    toChar i
+        | i < 26 = toEnum $ i + fromEnum 'A'
+        | i < 52 = toEnum $ i + fromEnum 'a' - 26
+        | otherwise = toEnum $ i + fromEnum '0' - 52
 
 -- | A tuple containing both the POST parameters and submitted files.
 type RequestBodyContents =