Make warning names more consistent

Sven Panne points out that current warning naming conventions used in GHC are rather inconsistent. Since we are moving away from -fwarn- to -W it would be nice if we could also try to fix up the names themselves.

We do, however, want to make sure that transition isn't more painful for users than necessary. For instance, the user should be able to get useful feedback from the compiler on what warning flags have changed with s/-fwarn-/-W/

To this end I recommend the following,

• We write down a consistent vocabulary for warning flag names

• We keep -fwarn- flags as they are currently

• We keep the inconsistently named -W flags corresponding to these

  -fwarn- flags

• We add consistently named -W flags alongside these

• We set a timeline for deprecating the inconsistent forms (perhaps in three or four releases)