We can remove several packages from the expected failures list due
to recent developments:

* We can remove `hashtables`, `http2`, `statistics`, and
  `unicode-transforms` from the expected failures for 8.8 now that
  the fix for ghc/ghc#16288 has been backported to the `ghc-8.8`
  branch.
* We can remove `singletons` and `vinyl` from the expected failures
  for `master` now that ghc/ghc!2102 has landed. The `vinyl-0.12.0`
  patch needs to be updated since it uses `TypeFamilyDependencies`
  in such a way that requires `UndecidableInstances`. (Note that
  `singletons` already enabled `UndecidableInstances` in all the
  right places, so no further action is required for that library.)