add warning for the Prelude being imported implicitly

[Isaac Dupree]

For those who don't like the implicit Prelude (but have an existing codebase to work with, or want to stay compatible with Haskell standards). The only ways to keep the warning from triggering, once enabled, are -fno-implicit-prelude/LANGUAGE NoImplicitPrelude, and/or at least one import ... Prelude ... line appearing in the module.

Proposed flag name: -fwarn-implicit-prelude

Proposed "on" status: not even on with -Wall, must be enabled explicitly

[simonpj]

Seems plausible to me, and should be easy to implement, so feel free to submit a patch. The guidelines are here;  http://hackage.haskell.org/trac/ghc/wiki/WorkingConventions#Howtosubmitapatchforanewfeature.

Thanks

Simon

[Isaac Dupree]

Implemented by me, patch applied by Simon PJ.