Explicit export set for Expander.TC

This makes the code easier to understand and improves GHC warnings.

src/Expander/TC.hs

@@ -1,7 +1,7 @@
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE MultiWayIf #-}
 {-# LANGUAGE ViewPatterns #-}
-module Expander.TC where
+module Expander.TC (unify, freshMeta, inst, specialize, varType, makeTypeMetas, generalizeType) where
 
 import Control.Lens hiding (indices)
 import Control.Monad.Except