Update effect-handlers to GHC 9.4.x

[ezrakilty]

I'm interested in using effect-handlers for a project, and I found it doesn't work with my somewhat-recent GHC.

In this initial version, I haven't completely updated it. I still get some errors which look like

TopLevel.hs:17:10: error: Not in scope: type variable ‘a’
   |
17 | [handler|
   |          ^...

and

TopLevel.hs:24:12: error:
    The Name ‘handler_ah9P’ is not in scope.
    Suggested fix:
      If you bound a unique Template Haskell name (NameU)
      perhaps via newName,
      then -ddump-splices might be useful.
   |
24 | [operation|forall a.Io :: IO a -> a|]
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^

and

TopLevel.hs:24:12: error: Not in scope: type variable ‘a’
   |
24 | [operation|forall a.Io :: IO a -> a|]
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^

Which seem a little perplexing because I believe a to be in scope through a forall binder, including when I use -ddump-splices to show the expanded code. But I have little context for the implementation to work with, so I'm hoping someone closer to the implementation can help me finish this.