Use Void type in function signTx for added safety.

This function is currently only used in contexts where all change outputs
have been assigned with addresses and are included in the set of ordinary
outputs. We use the 'Void' type here to prevent callers from accidentally
passing change values into this function.

lib/core/src/Cardano/Wallet.hs

@@ -406,6 +406,8 @@ import Data.Type.Equality
     ( (:~:) (..), testEquality )
 import Data.Vector.Shuffle
     ( shuffle )
+import Data.Void
+    ( Void )
 import Data.Word
     ( Word16, Word64 )
 import Fmt
@@ -1622,7 +1624,7 @@ signPayment ctx wid argGenChange mkRewardAccount pwd md cs = db & \DBLayer{..} -
 
 -- | Very much like 'signPayment', but doesn't not generate change addresses.
 signTx
-    :: forall ctx s t k input output change.
+    :: forall ctx s t k input output.
         ( HasTransactionLayer t k ctx
         , HasDBLayer s k ctx
         , HasNetworkLayer t ctx
@@ -1638,7 +1640,11 @@ signTx
     -> WalletId
     -> Passphrase "raw"
     -> Maybe TxMetadata
-    -> UnsignedTx input output change
+    -- This function is currently only used in contexts where all change outputs
+    -- have been assigned with addresses and are included in the set of ordinary
+    -- outputs. We use the 'Void' type here to prevent callers from accidentally
+    -- passing change values into this function:
+    -> UnsignedTx input output Void
     -> ExceptT ErrSignPayment IO (Tx, TxMeta, UTCTime, SealedTx)
 signTx ctx wid pwd md (UnsignedTx inpsNE outsNE _change) = db & \DBLayer{..} ->
     withRootKey @_ @s ctx wid pwd ErrSignPaymentWithRootKey $ \xprv scheme -> do