add double colon and double arrow symbols (-fglasgow-exts)

ghc/compiler/parser/Lexer.x

@@ -560,6 +560,8 @@ reservedSymsFM = listToUFM $
 
 #if __GLASGOW_HASKELL__ >= 605
        ,("λ",	ITlam,          bit glaExtsBit)
+       ,("∷",   ITdcolon,       bit glaExtsBit)
+       ,("⇒",   ITdarrow,	bit glaExtsBit)
        ,("∀",	ITforall,	bit glaExtsBit)
        ,("→",   ITrarrow,	bit glaExtsBit)
        ,("←",   ITlarrow,	bit glaExtsBit)