Enabling TypeHoles by default

[Herbert Valerio Riedel]

On 2014-01-15 at 03:24:29 +0100, Austin Seipp wrote:
> I'll submit a change to Cabal to remove its awareness of TypeHoles as
> an extension.

...actually, I forgot that 'TypeHoles' actually never made into Cabal
when we added missing language-extensions tokens, as Duncan noticed in
[1], that

| We didn't add TypeHoles on the theory that it would not be used in
| released packages. However on reflection it would be better to add it
| as a registered extension but to have an explicit check for it not
| being used in distributed packages (e.g. like we complain about
| -Werror).


Luckily, this turns my comments regarding updating Cabal & Hackage into
a non-issue :-)


 [1]: https://ghc.haskell.org/trac/ghc/ticket/8176#comment:6

Cheers,
  hvr