Warning on tabs by default (#9230) for GHC 7.10

[Austin Seipp]

Hi all,

Please see here:

https://phabricator.haskell.org/D255 and
https://ghc.haskell.org/trac/ghc/ticket/9230

Making tabs warn by default has been requested many times before, and
now that the compiler is completely detabbed, this should become
possible to enable easily, and we can gradually remove warnings from
everything else.

Unless someone has huge complaints or this becomes a gigantic
bikeshed/review (bike-review), please let me know - I would like this
to go in for 7.10.

-- 
Regards,

Austin Seipp, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com/