Remove the Unique library, which is no longer in use

compiler/Unique.hs

@@ -1,19 +0,0 @@
-{-# LANGUAGE GeneralizedNewtypeDeriving #-}
-module Unique (guid, set, run, runAt, Unique) where
-
-import Control.Monad.State (evalState, State, get, put)
-
--- Wrapper around State monad.
-newtype Unique a = U { runU :: State Int a }
-    deriving (Monad)
-
--- Get the next GUID, incrementing the counter.
-guid :: Unique Int
-guid = U $ do n <- get
-              put (n + 1)
-              return n
-
-set n = U (put n)
-
-run = runAt 0
-runAt n x = evalState (runU x) n