Drop mtl in favor of transformers

[#15] mtl 2.3 has a botched Control.Monad.Except export. We could work
around it with e.g. CPP, but luckily, we can also just use transformers
directly instead.

aeson-commit.cabal

@@ -30,10 +30,10 @@ library
   default-language: Haskell2010
   ghc-options:      -Wall -Wno-name-shadowing
   build-depends:
-      aeson  >=1.4  && <2.1
-    , base   >=4.10 && <5
-    , mtl    >=2.2  && <3
-    , text   >=1.2  && <2
+      aeson         >=1.4  && <2.1
+    , base          >=4.10 && <5
+    , text          >=1.2  && <2
+    , transformers  >=0.5  && <0.7
 
 test-suite tasty
   type:             exitcode-stdio-1.0

src/Data/Aeson/Commit.hs

@@ -41,9 +41,11 @@ module Data.Aeson.Commit
 where
 
 import Control.Applicative (Alternative (..))
-import Control.Monad.Except
+import Control.Monad.Trans.Class (lift)
+import Control.Monad.Trans.Except
 import Data.Aeson.Types
 import Data.Void (Void, absurd)
+
 #if MIN_VERSION_aeson(2,0,0)
 import qualified Data.Aeson.Key as Key
 #else