Added HasCallStack constraint for trace functions (#55)

* added HasCallStack constraint for trace functions, for issue #39

* unpack string on 7.6

src/CallStack.hs

@@ -0,0 +1,18 @@
+{-# LANGUAGE CPP #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE ImplicitParams #-}
+{-# LANGUAGE ConstraintKinds #-}
+
+module CallStack
+( HasCallStack
+) where
+
+#if MIN_VERSION_base(4,9,0)
+import GHC.Stack (HasCallStack)
+#elif MIN_VERSION_base(4,8,1)
+import qualified GHC.Stack
+type HasCallStack = (?callStack :: GHC.Stack.CallStack)
+#else
+import GHC.Exts (Constraint)
+type HasCallStack = (() :: Constraint)
+#endif