Add TypeOperators to default-extensions (#112)

* Fixes #106

Author: spl

dlist.cabal

@@ -54,6 +54,7 @@ library
     exposed-modules:    Data.DList.DNonEmpty
     other-modules:      Data.DList.DNonEmpty.Internal
   default-language:     Haskell2010
+  default-extensions:   TypeOperators
   ghc-options:          -Wall
   if impl(ghc >= 8.0)
     ghc-options:        -Wcompat