replace usage of 'mtl' with 'transformers'

There's no strong feeling about using one or another. Transformers is
slightly simpler as it doesn't introduce any of the type-classes and
functional dependencies that 'mtl' adds. Though, we only use the power
of 'transformers' so far, and thus, for consistency at least, let's
stick with this one and only pull 'mtl' if we feel it becomes a need

cardano-wallet.cabal

@@ -49,7 +49,6 @@ library
     , http-client
     , http-media
     , memory
-    , mtl
     , say
     , servant
     , servant-client
@@ -105,7 +104,6 @@ test-suite unit
     , fmt
     , hspec
     , memory
-    , mtl
     , process
     , QuickCheck
     , text

src/Cardano/NetworkLayer.hs

@@ -17,10 +17,10 @@ import Cardano.Wallet.Primitive
     ( Block (..), BlockHeader (..), Hash (..), SlotId (..) )
 import Control.Concurrent
     ( threadDelay )
-import Control.Monad.Except
-    ( ExceptT, runExceptT )
 import Control.Monad.IO.Class
     ( MonadIO, liftIO )
+import Control.Monad.Trans.Except
+    ( ExceptT, runExceptT )
 import Data.Time.Units
     ( Millisecond, toMicroseconds )
 import Fmt

src/Cardano/NetworkLayer/HttpBridge.hs

@@ -29,8 +29,10 @@ import Cardano.Wallet.Primitive
     ( Block (..), BlockHeader (..), Hash (..), SlotId (..) )
 import Control.Exception
     ( Exception (..) )
-import Control.Monad.Except
-    ( ExceptT (..), lift, runExceptT, throwError )
+import Control.Monad.Trans.Class
+    ( lift )
+import Control.Monad.Trans.Except
+    ( ExceptT (..), runExceptT, throwE )
 import Crypto.Hash
     ( HashAlgorithm, digestFromByteString )
 import Crypto.Hash.Algorithms
@@ -240,7 +242,7 @@ hashToApi'
     -> ExceptT HttpBridgeError m (Api.Hash algorithm b)
 hashToApi' h = case hashToApi h of
     Just h' -> pure h'
-    Nothing -> throwError
+    Nothing -> throwE
         $ BadResponseFromNode "hashToApi: Digest was of the wrong length"
 
 -- | Creates a cardano-http-bridge API with the given connection settings.

test/unit/Cardano/NetworkLayer/HttpBridgeSpec.hs

@@ -11,8 +11,10 @@ import Cardano.Wallet.Primitive
     ( Block (..), BlockHeader (..), Hash (..), SlotId (..), slotsPerEpoch )
 import Control.Monad.Catch
     ( MonadThrow (..) )
-import Control.Monad.Except
-    ( lift, runExceptT, throwError )
+import Control.Monad.Trans.Class
+    ( lift )
+import Control.Monad.Trans.Except
+    ( runExceptT, throwE )
 import Data.Word
     ( Word64 )
 import Test.Hspec
@@ -131,7 +133,7 @@ mockHttpBridge logLine firstUnstableEpoch tip = HttpBridge
         if ep < firstUnstableEpoch then
             pure $ mockEpoch ep
         else
-            throwError $
+            throwE $
                 "mock epoch " ++ show ep ++ " > firstUnstableEpoch " ++
                 show firstUnstableEpoch